Implement in-memory remotes for `update_remote_mirror` [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

The update_remote_mirror functions is currently using on-disk remotes by first calling remote_mirror.ensure_remote! followed by the call to update_repository. Gitaly is phasing out support for on-disk remotes though in favor of in-memory remotes, where the remote is specified as part of the request itself.

Implement the ability to use in-memory remotes for this RPC. For now, this is implemented behind a feature flag.

Part of: gitaly#1773 (closed), gitaly#1774 (closed) Feature flag rollout: #333517 (closed)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.