Add support for a custom commit message when cherry-picking commits using the API

What does this MR do?

This adds support for specifying a custom commit message when cherry-picking a commit using the API.

This is needed to resolve https://gitlab.com/gitlab-org/release-tools/-/issues/500, as Release Tools needs a way to cherry-pick commits while retaining the Git trailers we use for generating changelogs.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.