Add endpoint to update a git submodule reference

What does this MR do?

This MR adds a new endpoint (api/v4/projects/:id/repositories/submodules/:submodule) to update the submodule reference stored in a repository.

gitaly!936 (merged), gitaly-proto!233 (merged) and gitaly-proto!237 (merged) are also needed for this MR.

Why was this MR needed?

Some users that automate their workflows needed an endpoint to automatically update all the related projects where the submodule is included.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/41213