Update API: add search param to branches

What does this MR do?

Add possibility to pass search param to Branches API.

Closes: https://gitlab.com/gitlab-org/gitlab-ce/issues/42712

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

Yes. Please make sure that user input as a param is truly validated and that SQL injection is not possible with 'search' param.

Why was this MR needed?

To improve the usability of Branches API endpoint by allowing search of the branches.

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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/42712