Implement asynchronous transfer functionality for groups and projects using the existing `transfer_in_progress` state from the `Namespaces::Stateful` concern. This work will move the transfer processing from synchronous controller actions to background workers, preventing timeouts for large transfers. All work should be behind the `groups_and_projects_async_transfer` feature flag. This issue is to implement the gitlab~2492649 work. - https://gitlab.com/gitlab-org/gitlab/-/blob/4a7f47527840c2e4686d4475e2eeeaed1beb843a/app/controllers/groups_controller.rb#L228 - https://gitlab.com/gitlab-org/gitlab/-/blob/4a7f47527840c2e4686d4475e2eeeaed1beb843a/app/controllers/projects_controller.rb#L130