Add rootClusterAgents GraphQL field for workspace creation

Context

The following discussion from !118265 (merged) should be addressed:

• @pslaughter started a discussion: (+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.