Geo repository sync over HTTPS

What does this MR do?

Adds a HTTPS repository sync mechanism for Geo

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

The intention, I think, should be to deprecate SSH authentication in 10.2. If we receive feedback that it's essential to maintain support, we can finish #1255 (closed) and un-deprecate it at that point. If not, we can remove it a few releases later and leave HTTPS as the only supported mechanism.

Why was this MR needed?

SSH requires extra authentication data that is difficult to manage manually. Automated management is possible, but only worth it if there is a significant benefit to SSH over HTTPS - and we've not found one yet.

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 Frontend
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #3341 (closed)