Part of epic https://gitlab.com/groups/gitlab-org/-/epics/16678 ### Problem https://gitlab.com/gitlab-org/gitlab/-/issues/517139+ adds a new field `allow_fork_sync` to `protected_branches` table. We want to allow users to configure its state on UI. ### Proposal Add a functioning toggle to branch rules create/edit pages. See spike: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/176718/diffs?commit_id=f184977ca5b70a69fb3cfc982dc039f319217285