Support creation in entity reference sub-field

Problem/Motivation

When a sub-field is of type "entity Reference", it would be very useful to be able to create a new instance of the referenced type, not just referencing.

Proposed resolution

Adding support for creation of referenced items like we have in Entity Reference fields not embedded into a Custom Field.

User interface changes

A new option "Create referenced entities if they don't already exist"

API changes

N/A