[Gateway] Implement gateway unbonding period (#1062)

## Summary

This pull request introduces a **Gateway unbonding period**, preventing
gateways from immediately unstaking after a `MsgUnstakeGateway` message
is committed.

### Primary Changes:
- Updated the `UnstakeGateway` message handler to mark a gateway as
unbonding instead of removing it.
- Added `EventGatewayUnbondingBegin` and `EventGatewayUnbondingEnd`
events.
- Implemented the `EndBlockerUnbondGateways` keeper method to finalize
gateways unbonding.

### Secondary changes:
- Introduced the `GatewayUnbondingPeriodSessions` shared governance
parameter.
- Updated unit and E2E tests to reflect these changes.

## Issue

- Issue: #731 

## Type of change

Select one or more from the following:

- [x] New feature, functionality or library
- [x] Consensus breaking; add the `consensus-breaking` label if so. See
#791 for details
- [ ] Bug fix
- [ ] Code health or cleanup
- [ ] Documentation
- [ ] Other (specify)

## Sanity Checklist

- [x] I have updated the GitHub Issue `assignees`, `reviewers`,
`labels`, `project`, `iteration` and `milestone`
- [ ] For docs, I have run `make docusaurus_start`
- [x] For code, I have run `make go_develop_and_test` and `make
test_e2e`
- [ ] For code, I have added the `devnet-test-e2e` label to run E2E
tests in CI
- [ ] For configurations, I have update the documentation
- [ ] I added TODOs where applicable

---------

Co-authored-by: Daniel Olshansky <[email protected]>