Declare merge queue barrier files in your configuration

When you split your merge queue into parallel scopes with file patterns, some files — CI workflows, build files, shared tooling — affect everything. Add a barrier_files list to your scopes configuration and any pull request changing a matching file automatically becomes a merge-queue barrier: it runs on its own and serializes the queue at that point, instead of merging in parallel with other scopes. Previously this could only be triggered through the scopes API.