Add feature flag to use gitaly-ssh mirroring when cloning internal repos

What does this MR do?

Add feature flag to use gitaly-ssh mirroring when cloning internal repos.

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

No.

Why was this MR needed?

For gitaly#554 (closed) we need to stop working with disk paths

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
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes gitaly#739 (closed)