Allow replying to an individual note in the API

What does this MR do?

This allows replying to a discussion that represents an individual note, but is 'replyable', from the API. You can already do this from the UI.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/61392.