Implement Gitaly refs client
What does this MR do?
Implement Gitaly refs client with initial operations default_branch_name, branch_names and tag_names.
Are there points in the code the reviewer needs to double check?
No
Why was this MR needed?
Provide client code for gitaly#65 (closed)
Does this MR meet the acceptance criteria?
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Changelog entry added 
- [ ] Documentation created/updated
- [ ] API support added
- Tests
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Added for this feature/bug 
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All builds are passing 
 
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Conform by the merge request performance guides 
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Conform by the style guides 
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Branch has no merge conflicts with master(if it does - rebase it please)
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Squashed related commits together