WIP: Add support for diff context in MR notes API #31104

What does this MR do?

This MR adds diff context to the notes api for diff notes.

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

• Are the api fields named correctly?
• How should I go about testing this?
• Am I extracting :commit_id correctly? Its not mentioned in #31104 (moved), but seems necessary so that outdated notes can be identified.

Why was this MR needed?

To address #31104 (moved)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #31104 (moved)