Add rootClusterAgents GraphQL field for workspace creation
## Context The following discussion from !118265 should be addressed: - [ ] @pslaughter started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/118265#note_1362340378): (+2 comments) > **question (non-blocking):** To improve what's going on with the double query behavior, have we considered adding a **`rootClusterAgents`** (similar to `rootStorageStatistics`) in the BE which would pull from the current group, if it's a root group, or find the root group and pull from that one. > > This way we'd only have to talk to the BE once.
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