imp(ibc)!: refactor `Timestamp` type and distinguish timeout timestamp

[Farhad-Shabani]

Closes: #180 Closes: #1296 Closes: #1306

Description

This PR introduces following improvements:

• Separates the Timestamp definitions used by the host from those used for timeout purposes. The host timestamp is mandatory, while a None value for the timeout timestamp indicates the absence of a timeout.
• Implements a direct, more efficient conversion to/from protobuf timestamps, eliminating the previous redirection through tendermint::Time, which was also intended for tendermint_proto::google::Timestamp instead of ibc_proto::google::Timestamp.
• Decouples timestamp definition from tendermint::Time, laying the groundwork for fully decoupling ibc_primitive from the tendermint dependency, making it chain-agnostic.
• Fixes an issue where tendermint::Time could produce negative UNIX timestamps, causing a panic when calling Timestamp::nanoseconds().
• Unifies all timestamp-related errors under a single TimestampError type
• Removes the error variant for Timestamp::from_nanoseconds, as it never fails.

  ---

PR author checklist:

• [x] Added changelog entry, using unclog.
• [X] Added tests.
• [X] Linked to GitHub issue.
• [X] Updated code comments and documentation (e.g., docs/).
• [X] Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• [ ] Reviewed Files changed in the GitHub PR explorer.
• [ ] Manually tested (in case integration/unit/mock tests are absent).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 76.13636% with 63 lines in your changes missing coverage. Please review.

Project coverage is 67.07%. Comparing base (7e39192) to head (ac1ec73).

Files	Patch %	Lines
ibc-primitives/src/types/timestamp.rs	76.98%	29 Missing :warning:
...-core/ics04-channel/types/src/timeout/timestamp.rs	80.00%	13 Missing :warning:
ibc-clients/ics07-tendermint/types/src/header.rs	0.00%	6 Missing :warning:
ibc-testkit/src/hosts/tendermint.rs	50.00%	6 Missing :warning:
ibc-core/ics04-channel/types/src/events/mod.rs	20.00%	4 Missing :warning:
ibc-apps/ics20-transfer/types/src/msgs/transfer.rs	0.00%	2 Missing :warning:
...pps/ics721-nft-transfer/types/src/msgs/transfer.rs	0.00%	2 Missing :warning:
ibc-core/ics04-channel/types/src/timeout/height.rs	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1307 +/- ## ========================================== - Coverage 67.08% 67.07% -0.01% ========================================== Files 226 227 +1 Lines 23336 23383 +47 ========================================== + Hits 15654 15685 +31 - Misses 7682 7698 +16 ```

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