feat!: only distribute rewards to validators that have been validating a consumer chain for some time

Description

Closes: (partially) https://github.com/cosmos/interchain-security/issues/1926 and https://github.com/informalsystems/interchain-security/issues/27

Problem A validator might opt in to a consumer chain simply to retrieve rewards without validating the consumer chain for too long.

Solution We introduce a height field in ConsumerValidators. This height corresponds to the block height of the provider chain when the consumer validator first became a consumer validator. The set height of a ConsumerValidator remains stable as long as the validator remains a consumer validator. The height of a ConsumerValidator only resets if a validator stops being a consumer validator and later becomes again a consumer validator.

We introduce a new param NumberOfEpochsToStartReceivingRewards and a validator can only start receiving rewards after being a consumer validator for NumberOfEpochsToStartReceivingRewards epochs or after NumberOfEpochsToStartReceivingRewards * BlocksPerEpoch blocks.

Note that at the beginning of a provider chain (e.g., when the block height of the provider is still small), no validator could have been a consumer validator for more than NumberOfEpochsToStartReceivingRewards epochs and hence in this case we distribute rewards to validators, even though they might have not been validating the consumer chain for more than a single epoch.

Finally, note that in a consumer chain that just starts, no validator would receive rewards during the first NumberOfEpochsToStartReceivingRewards epochs. During this time, any rewards sent to the provider from the consumer chain through IBC transfers will be allocated to the community pool.

Testing Two main things are being tested:

the height of a consumer validator is not being reset if the validator remains a consumer validator (unit test TestQueueVSCPacketsDoesNotResetConsumerValidatorsHeights);
that validators only receive rewards if they have been consumer validators for some time (integration test TestAllocateTokensToConsumerValidatorsWithDifferentValidatorHeights).

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and please add links to any relevant follow up issues.

I have...

[x] Included the correct type prefix in the PR title
[ ] Added ! to the type prefix if the change is state-machine breaking
[ ] Confirmed this PR does not introduce changes requiring state migrations, OR migration code has been added to consumer and/or provider modules
[x] Targeted the correct branch (see PR Targeting)
[ ] Provided a link to the relevant issue or specification
[ ] Followed the guidelines for building SDK modules
[ ] Included the necessary unit and integration tests
[x] Added a changelog entry to CHANGELOG.md
[x] Included comments for documenting Go code
[ ] Updated the relevant documentation or specification
[ ] Reviewed "Files changed" and left comments if necessary
[ ] Confirmed all CI checks have passed
[ ] If this PR is library API breaking, bump the go.mod version string of the repo, and follow through on a new major release

Reviewers Checklist

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I have...

[ ] confirmed the correct type prefix in the PR title
[ ] confirmed ! the type prefix if the change is state-machine breaking
[ ] confirmed this PR does not introduce changes requiring state migrations, OR confirmed migration code has been added to consumer and/or provider modules
[ ] confirmed all author checklist items have been addressed
[ ] reviewed state machine logic
[ ] reviewed API design and naming
[ ] reviewed documentation is accurate
[ ] reviewed tests and test coverage

Summary by CodeRabbit

New Features
- Implemented a new reward distribution mechanism where validators receive rewards only after validating for a specific number of blocks.
- Introduced a new parameter, NumberOfEpochsToStartReceivingRewards, to configure reward eligibility.
Documentation
- Updated documentation to include a warning about validator rewards eligibility and revised instructions for withdrawing rewards.
Tests
- Added tests to validate the new reward eligibility criteria and ensure accurate token allocation based on validator heights.
Migration
- Implemented functions for migrating provider chain parameters and ensuring compatibility with the new reward distribution system.

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Commits
Files that changed from the base of the PR and between 395409128f29dee730d5d9103f0cf18067bb1b7b and d28aba5493be187c2654794f46f9f69184c1d76d.

Walkthrough

This update introduces a new parameter, NumberOfEpochsToStartReceivingRewards, which delays reward distribution to validators until they have been validating for a specified number of blocks. The integration also encompasses adjustments to documentation, tests, and migration scripts to incorporate this parameter and ensure the system correctly reflects validation duration in reward allocations.

Changes

Files	Change Summary
`.changelog/unreleased/improvements/provider/...`	Added a summary for changes related to distributing rewards to long-term validating validators.
`.changelog/unreleased/state-breaking/provider/...`	Summarized state-breaking changes related to reward distribution based on validation duration.
`docs/docs/validators/withdraw_rewards.md`	Updated title and added warning about validator reward eligibility based on validation duration.
`proto/interchain_security/ccv/provider/v1/provider.proto`	Introduced new fields in `Params` and `ConsumerValidator` messages for reward eligibility and validator height.
`tests/integration/distribution.go`	Renamed and added functions to test reward allocation among validators with different heights.
`x/ccv/provider/keeper/distribution.go`	Added `IsEligibleForConsumerRewards` function and integrated it into reward allocation and total power computation.
`x/ccv/provider/keeper/distribution_test.go`	Added tests for `IsEligibleForConsumerRewards` and setting of `BlocksPerEpoch` parameter in total power computation.
`x/ccv/provider/keeper/relay_test.go`	Added a test to ensure consumer validator heights are maintained during packet queuing.
`x/ccv/provider/migrations/migrator.go`	Updated `Migrate5to6` function to call new migration functions for parameters and minimum power.
`x/ccv/provider/migrations/v6/migration_test.go`	Added test to verify parameter migration related to epochs.
`x/ccv/provider/migrations/v6/migrations.go`	Added new function for migrating provider chain parameters.
`x/ccv/provider/types/params.go`	Added and validated new parameter `NumberOfEpochsToStartReceivingRewards` in `Params` struct along with default values and key constants.

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cosmos / interchain-security