Field definition container: include ER fields as well as URIs

Problem/Motivation

Since #2462293: Support HAL, the normalisations of field definitions for entity reference fields (with content entity target types) are replaced with URI list definitions. This is an obstacle to recreating the field definitions and comparing to existing field definitions.

Proposed resolution

Do not replace the field definitions with URI list definitions. Instead, include both. Field definitions could be available in the property 'fields' as before the HAL commit, and ER URI definitions under a new 'references' property.

Remaining tasks

User interface changes

API changes