Merge Request Discussions API: accept commit_id to create discussion on commit

What does this MR do?

When creating a new thread on a merge request by using the MR Discussions API, the thread is always on the merge request's entire diff.

This patch allows passing the commit_id parameter to associate the thread with the desired commit.

Here is a screenshot of the results. Both comments were created using the API. The second comment was created after my changes, and thus contains the desired started a thread on commit 555c8706.

Before

After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team