Follow-up from "Generate policies and jobs from the new SSOT"

The following discussion from !2572 (merged) should be addressed:

• @godfat-gitlab started a discussion:

  It looks like we can potentially merge GroupStruct and StageStruct into something like DefinitionStruct, and move DEFAULT_ASSIGNEE_FIELDS and MENTION_FIELDS into it because they look the same for both groups and stages, and this new DefinitionStruct can be served as the shared base.

  Given this merge request is already pretty large, I think we can do this in the next iteration if we want.