Create wrapper to use Gitaly DiffStats RPC

What does this MR do?

• This creates a wrapper method on Gitaly commit client service in order to use DiffNumStat RPC
• It doesn't add any additional code to make GitLab use the RPC for now
• This will be used in upcoming releases

Proto MR: gitaly-proto!206 (merged) Server MR: gitaly!808 (merged)

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

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 to the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conform to 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?

Closes gitaly#1147 (closed)