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/controllers/namespaces/git_http_client_controller.rb

@@ -16,8 +16,8 @@ class Namespaces::GitHttpClientController < Namespaces::ApplicationController
   skip_around_action :set_session_storage
   skip_before_action :verify_authenticity_token
 
-  before_action :authenticate_user
   before_action :project
+  before_action :authenticate_user
 
   private
 
@@ -48,8 +48,7 @@ def authenticate_user
         send_final_spnego_response
         return # Allow access
       end
-    elsif project && download_request? && http_allowed? && Guest.can?(:download_code, project)
-
+    elsif http_allowed?
       @authentication_result = Gitlab::Auth::Result.new(nil, project, :none, [:download_code])
 
       return # Allow access
@@ -79,7 +78,7 @@ def project
   end
 
   def parse_repo_path
-    @project, @repo_type, @redirected_path, @subject = Gitlab::RepoPath.parse("#{params[:namespace_id]}/#{params[:repository_id]}")
+    @subject, @project, @repo_type, @redirected_path = Gitlab::RepoPath.parse("#{params[:namespace_id]}/#{params[:repository_id]}")
   end
 
   def render_missing_personal_access_token
@@ -113,7 +112,9 @@ def ci?
   end
 
   def http_allowed?
-    Gitlab::ProtocolAccess.allowed?('http')
+    Gitlab::ProtocolAccess.allowed?('http') &&
+    download_request? &&
+    (project && Guest.can?(:download_code, project) || @subject.is_a?(PersonalSnippet))
   end
 end