Geo: backend support to allow Secondary node to push to a Primary as push proxying

What does this MR do?

Implements backend support to authorize Secondary nodes to push code to primary when authenticated with Geo JWT tokens.

• Allow Geo JWT to satisfy GitAccess
• Allow push_code privileges when authenticated via Geo JWT
• Passes gl_id and gl_username along to Workhorse

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

• Need Changelog entry?
• Need documentation change?
• Suitable tests exist
• FIXMEs addressed

❗ Works with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21038 - Allow auth via JWT (always false in CE)

Why was this MR needed?

To facilitate the proxying of SSH git push requests sent to the primary, from the secondary https://gitlab.com/gitlab-org/gitlab-ee/issues/6533

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
• [ ] Conform by the database 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-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/6533