Support allowed_values_function

I gave this a try and it was pretty simple. Opened a merge request for review...

See attached before and after of a lab_test_type field that I am testing this with.

@Wim Leers: I'm not sure if this covers the case that you described. Maybe that requires a different change, and thus should be a separate issue? Is it directly related to allowed_values_function?

Attached is a patch for use in composer.json with cweagans/composer-patches while we wait for this to be reviewed/merged. It is identical to the MR.

I opened a new issue which replaces the need for this one, and adds support for even more field types with "allowed values": ✨ Use OptionsProviderInterface::getPossibleOptions() for allowed field values (anyOf / oneOf) Needs review

I'm going to close this as "won't fix" so we can turn attention to that one instead.