API: Multi-file commit

What does this MR do?

Adds a new endpoint to the GitLab API allowing for commiting multiple files at once

Are there points in the code the reviewer needs to double check?

• When creating the tests I noticed that the call returns on the first validation error, should I update it so that the entire call is validated and return all errors, or is failing at the first issue acceptable?
• What should be returned? The current file APIs return the branch and file_path. This call can have multiple files so it's not as clear, currently it just returns the branch.

Why was this MR needed?

When using the API to work with the repository, the commit log gets cluttered with one commit per file.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #17350 (closed)