Add Group ProjectNamespace Shared Policy

app/policies/group_policy.rb

 # frozen_string_literal: true
 
-class GroupPolicy < NamespacePolicy
+class GroupPolicy < Namespaces::GroupProjectNamespaceSharedPolicy
   include FindGroupProjects
 
   desc "Group is public"

app/policies/namespaces/group_project_namespace_shared_policy.rb

+# frozen_string_literal: true
+
+module Namespaces
+  class GroupProjectNamespaceSharedPolicy < ::NamespacePolicy
+    # Nothing here at the moment, but as we move policies from ProjectPolicy to ProjectNamespacePolicy,
+    # anything common with GroupPolicy but not with UserNamespacePolicy can go in here.
+    # See https://gitlab.com/groups/gitlab-org/-/epics/6689
+  end
+end

app/policies/namespaces/project_namespace_policy.rb

 # frozen_string_literal: true
 
 module Namespaces
-  class ProjectNamespacePolicy < NamespacePolicy
+  class ProjectNamespacePolicy < Namespaces::GroupProjectNamespaceSharedPolicy
     # For now users are not granted any permissions on project namespace
     # as it's completely hidden to them. When we start using project
     # namespaces in queries, we will have to extend this policy.