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/gl_repository/repo_type.rb

@@ -6,13 +6,21 @@ class RepoType
       attr_reader :name,
                   :access_checker_class,
                   :repository_accessor,
+                  :repositorable_accessor,
                   :project_accessor,
                   :suffix
 
-      def initialize(name:, access_checker_class:, repository_accessor:, project_accessor: nil, suffix: nil)
+      def initialize(
+        name:,
+        access_checker_class:,
+        repository_accessor:,
+        repositorable_accessor: default_repositorable_accessor,
+        project_accessor: nil,
+        suffix: nil)
         @name = name
         @access_checker_class = access_checker_class
         @repository_accessor = repository_accessor
+        @repositorable_accessor = repositorable_accessor
         @project_accessor = project_accessor
         @suffix = suffix
       end
@@ -26,6 +34,14 @@ def fetch_id(identifier)
         match[:id] if match
       end
 
+      def fetch_repositorable!(identifier)
+        id = fetch_id(identifier)
+
+        raise ArgumentError, "Invalid GL Repository \"#{identifier}\"" unless id
+
+        repositorable_accessor.call(id)
+      end
+
       def wiki?
         self == WIKI
       end
@@ -59,6 +75,12 @@ def valid?(repository_path)
           repository_path.match?(Gitlab::PathRegex.full_snippets_repository_path_regex)
         )
       end
+
+      private
+
+      def default_repositorable_accessor
+        -> (id) { Project.find_by_id(id) }
+      end
     end
   end
 end