API support for setting squash commit message during squash

What does this MR do?

Allows a client to set the squash commit message when squashing a merge commit through the API.

This is the API-specific component of work done in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24518 and can be merged into master once that MR has been merged.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/47149
• https://gitlab.com/gitlab-org/gitlab-ce/issues/56014

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #56014 (closed)