SearchKit: allow custom fields to be used in the JOIN conditions (like they're available in where)

As far as I can tell, the custom fields on the entity that is joined are not available for the JOIN conditions. But they are visible in the WHERE.

Here's a scenario: there's a custom field on a contact. Create a new SearchKit with primary being Address. Then JOIN the Contact. Add a condition for the join and search the contact's custom field. It's not available. Then check for that same custom field in the WHERE - it should be available.

This is useful when using GROUP BY and cannot use the WHERE. And where it would be inconvenient/impossible to add the field as a column and use HAVING. For example, if the added field has a transformation to COUNT then HAVING won't help.