CustomGroup - Add metadata about how a custom group relates to entity types

CustomGroup - Add metadata about how a custom group relates to entity types

Fixes dev/core#2905
Before: Hard coded, plus a very strange use of the 'description' field to store a callback function.

After: The 'grouping' field in the OptionValue for custom extends and custom type is used.
APIv4 getfields can then retrieve the necessary options.