Add branch existence check to the APIv4 branches via HEAD request

What does this MR do?

This MR adds a processing of the HEAD requests to the branches APIv4.

HEAD /projects/:id/repository/branches/:branch

By this requests, only check if a branch exists without loading a branch data from a repository.

Why was this MR needed?

Improve performance of API endpoints in case if a user wants just to check if a branch exists or not without loading anything else.

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

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/37159