[Gateway] Add missing stake event (#869)

## Summary

- Adds and emits a new `EventGatewayStaked` event to the gateway module.
- Refactors `EventGatewayUnstaked` to contain the entire gateway and
session end height.
- Adds the shared module keeper as a gateway keeper dependency.

## Issue

- #858

## Type of change

Select one or more from the following:

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

## Testing

- [ ] **Documentation**: `make docusaurus_start`; only needed if you
make doc changes
- [ ] **Unit Tests**: `make go_develop_and_test`
- [ ] **LocalNet E2E Tests**: `make test_e2e`
- [ ] **DevNet E2E Tests**: Add the `devnet-test-e2e` label to the PR.

## Sanity Checklist

- [x] I have tested my changes using the available tooling
- [x] I have commented my code
- [x] I have performed a self-review of my own code; both comments &
source code
- [ ] I create and reference any new tickets, if applicable
- [ ] I have left TODOs throughout the codebase, if applicable

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Co-authored-by: Redouane Lakrache <[email protected]>
Co-authored-by: Daniel Olshansky <[email protected]>
Co-authored-by: red-0ne <[email protected]>