Add Repository#set_config and #delete_config

What does this MR do?

Add methods to set and delete Git config keys via Gitaly.

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

Why was this MR needed?

When fetching repositories on a Geo secondary we set and delete config values around the FetchRemote RPC.

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
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database 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?

gitaly#1241 (closed)