Geo: Add support for "proxy to primary when there isn't a repo" for snippet repositories

The following discussion from !40017 (merged) should be addressed:

• @mkozono started a discussion: (+2 comments)

  question: I don't see the full picture yet so please bear with me.

  IIUC, this override is related to "proxy to primary when there isn't a repo": https://gitlab.com/gitlab-org/gitlab/-/blob/v13.4.1-ee/ee/lib/ee/gitlab/geo_git_access.rb

  The code in there has references to project or wiki but not snippet, so I assume it will take some work for snippets. If true, then would it work to leave this override and open a follow up to add support for "proxy to primary when there isn't a repo" for snippet repositories? If not, we need to cover this functionality in tests.