Resolve "Transparently proxy HTTP Git push from secondary to primary"

What does this MR do?

This MR adds the ability for Geo customers to push to a secondary and have the request redirected automatically to the primary without any configuration.

The vast majority of this change was spiked out by @mkozono so massive props to him 🙇

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

Documentation - interested in ideas on how / where we should document this new feature?

I added a feature toggle around the functionality. May not need it? If needed, is geo_push_to_secondary an appropriate name choice?

Also am not entirely happy / convinced with the test coverage as I simply added to what was there. Can probably be improved.

Why was this MR needed?

Existing Geo customers need to maintain a separate git origin in their config and use it to push code, so they would typically have an origin pointing to the secondary and origin-primary (or such) pointing to the primary to facilitate pushing.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [ ] API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • [ ] Has been reviewed by a UX Designer
  • [ ] Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • [ ] Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• [ ] If you have multiple commits, please combine them into a few logically organized commits by squashing them
• [ ] 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 #3912 (closed)