Adds `stake-tracker` pallet

[gpestana]

This PR adds the stake-tracker pallet without integrating it with the staking and bags lists pallets.

The stake-tracker pallet implements the OnStakingUpdate trait to listen to staking events and multiplexes those events to one or multiple types (e.g. pallets). Initially, the stake tracker pallet is used as a degree of indirection to maintain the target and voter semi-sorted lists up to date.

The final goal is to achieve a design where the targets and voters lists are updated at each relevant staking event through the stake-tracker pallet. The list reads in staking are performed directly through the SortedListProvider.

To finalize

• [x] Improve test coverage
• [ ] Consider a multi-block event processing to handle updates on slashes and rewards for both targets and voters (unconfirmed, perhaps on a follow-up PR).

Related and step towards https://github.com/paritytech/polkadot-sdk/issues/443

[gpestana]

bot rebase

[paritytech-processbot[bot]]

Rebased

[gpestana]

bot fmt

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[command-bot[bot]]

@gpestana Command "$PIPELINE_SCRIPTS_DIR/commands/fmt/fmt.sh" has finished. Result: https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/3324898 has finished. If any artifacts were generated, you can download them from https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/3324898/artifacts/download.

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs. Job name: test-linux-stable Logs: https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/3324915