Resolve "Geo selective sync by repository shard"

What does this MR do?

Adds a new type of selective sync - shard by shard. This is envisioned for use when rolling out Geo onto a larger installation. Being able to validate the setup with projects on a single shard allows the I/O load Geo places on the primary to be localised, preventing outages across many shards if the load turns out to be too high.

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

JS changes especially need validation. I am not amazing at JS :)

Why was this MR needed?

Requested by production in the context of the GCP Migration

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
• 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
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #4644 (closed)