Add Epochs to Address Unbonding Endpoint

[shellvish]

Context and purpose of the change

This PR adds a new field to the address unbonding endpoint. In particular, this helps identify which epoch an unbonding started in. It's a relatively trivial addition of code.

Author's Checklist

I have...

• [ X ] Run and PASSED locally all GAIA integration tests
• If the change is contentful, I either:
  • Added a new unit test OR
  • Added test cases to existing unit tests
• [ X ] OR this change is a trivial rework / code cleanup without any test coverage

If skipped any of the tests above, explain.

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• reviewed state machine logic
• reviewed API design and naming
• manually tested (if applicable)
• confirmed the author wrote unit tests for new logic
• reviewed documentation exists and is accurate

[github-actions]

This pull request has been automatically marked as stale because it has not had any recent activity. It will be closed if no further activity occurs. Thank you!

[sampocs]

lgtm!