Add routes to allow git actions on snippet repositories

What does this MR do?

In this MR we implement the following:

• Refactors the git_http routes because, for now, snippet repositories won't support LFS (mainly because lfs_enabled is a project feature and we have to think if we could enable it by default for personal snippets).
• It also updates the GitHttpController(s) and the SSH internal API to allow basic cloning/pushing/pulling operations for resources other than projects.

Refs #39176 (closed) Closes #205666 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

lib/gitlab/repository_cache.rb

@@ -7,7 +7,7 @@ class RepositoryCache
 
     def initialize(repository, extra_namespace: nil, backend: Rails.cache)
       @repository = repository
-      @namespace = "#{repository.full_path}" + (repository.project && ":#{repository.project.id}")
+      @namespace = "#{repository.full_path}" + (repository.project && ":#{repository.project.id}").to_s
       @namespace = "#{@namespace}:#{extra_namespace}" if extra_namespace
       @backend = backend
     end