APIv4 - Return id_field as part of Entity.get

APIv4 - Return id_field as part of Entity.get

All entities have a unique identifier field, usually named 'id'
but some entities the field is named something else.
e.g. Afform uses 'name' as the identifier.

This returns the name of the field as part of Entity.get,
and it's also available directly from each API class e.g. Contact::getInfo().