Merge request and commit discussions API

What does this MR do?

• Adds discussions API for merge requests and commits which allows to manage both discussion notes and also diff notes.
• This patch also adds diff_ref to MergeRequest entity which is not necessary for this feature itself, but I think there is currently not simple way how to get diff_refs values which are required when creating a DiffNote.
• noteable authorization check is moved directly into find_noteable method (so it's removed from some places in the existing API)

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

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug

What are the relevant issue numbers?

Closes #20901 (closed)