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
issue