Group Level Wiki: BE Add basic support for Git routes

Problem to solve

We need to able to address group wiki repositories from inside GitLab, so when editing wikis through our UI we can create commits.

Note: This issue is a child of the Group Level Wiki Implementation Epic &2214 (closed)

Proposal

Add all required Git routes, similar to the changes in !21739 (merged).

Not all of this is necessary yet, we'll focus on external access over SSH/HTTP in #207869 (closed).

Feature Flag

Any user-accessible routes need to be behind a group-level feature flag named group_wiki.

Tentative MR Breakdown

Only one MR is predicted for now.

Permissions and Security

Use the permissions established in #207872 (closed).

Documentation (remove if not applicable)

This issue does not require the creation of documentation.

Testing

Testing Approach

• Analyze the existing project wiki specific tests. Is it covered at the appropriate level?
• Adapt existing tests for project wikis to group wikis.

Success Criteria

• Group wiki repositories can be accessed from inside GitLab

Links / references