Add extra field support [follow up to #137]

[DuaelFr]

As seen in #137 there was not much to do to get this ready for commit. One year later, here we are :)

[manuel-adan]

I tested it with a fieldgroup pattern with an EVA field as source. I'm getting:

Fatal error: Maximum execution time of 30 seconds exceeded in /var/www/web/modules/contrib/ui_patterns/modules/ui_patterns_field_group/src/Utility/EntityFinder.php on line 29

It seems there are some kind of circular reference in the field tree.

[DuaelFr]

Rebased on the last 8.x-1.x Checked for D9 compatibility, no issue found.

[DuaelFr]

This PR has been moved to Drupal.org issue queue: https://www.drupal.org/project/ui_patterns/issues/3311088