API: Add endpoint to amend commits

Problem to solve

Client requests API endpoint to amend a commit

Further details

https://gitlab.zendesk.com/agent/tickets/104065

Proposal

What does success look like, and how can we measure that?

(If no way to measure success, link to an issue that will implement a way to measure this)

Client can utilize API endpoint to amend commits without cloning the entire repo locally.

Links / references