Add hook for checking field definition access

I will need to add a test to this, but would love feedback on whether the webform community think this is a good idea and whether this hook should touch logic in the results table as well as in the original function.

Also adding a patch here for Webform 6.2.x, as webform.api.php isn't using namespaced vars in that version.

Any access control improvement will need some test coverage.

liam morland → made their first commit to this issue’s fork.