Search API Group Relationships across entity types

This provides a group relation entity properties. One for each of the group enabled entity types. In a way, it sorta operates like a deriver but this is how Search API works.

@heddn does this work with search API to get group members and its user data?

It should, but you'd have to test that to confirm.

Trying a no-git re-roll.

Added it to a MR for easier follow up. Still need to be tested.