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

app/models/snippets/repository.rb

@@ -2,7 +2,8 @@
 
 module Snippets
   class Repository < ::Repository
-    # We still need to override project, defined in the parent class.
+    # We still need to override project because it is
+    # defined in the parent class.
     # At some point, the main class Repository will remove the project
     # reference and work only with subject
     delegate :project, to: :subject