Client Implementation: RefService::RefExists

Closes gitaly#452 (closed)

EE Companion: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2695

What does this MR do?

Delivers the RefService::RefExists endpoint

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

Why was this MR needed?

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

What are the relevant issue numbers?