Add an attributes bag class as a GitalyClient helper

Related to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15014#note_45331850

What does this MR do?

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Why was this MR needed?

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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
• Internationalization required/considered

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