Afform - support filters in custom contactRef & core entityRef fields

Afform - support filters in custom contactRef & core entityRef fields

Fixes dev/core#3425
Provides a common format for "filters" in both core and custom entityRef fields.
For example, the `employer_id` field now automatically filters `contact_type=Organization`.
Custom fields use the same format for filters (basically URL encoded format),
and APIv4 now reads them both and applies them when the field is included in an Afform.