Remove the restriction from RecursiveContextualValidator that prevents using custom groups.

Seems straight forward.

Given the info from #2820364-77: Entity + Field + Property validation constraints are processed in the incorrect order → , I'd be quite surprised if removing the LogicException would be sufficient to support composite validators.

If we're going to do this it would be good to have some test coverage to prove that it does work (or indeed, as #6 implies, doesn't work).