Add source and target branch filters to merge requests API

What does this MR do?

Adds source_branch and target_branch filters to the MR API and finder.

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

🤷

Why was this MR needed?

1. It's generally useful.
2. To help the VSCode plugin.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/42492.