Ability to specify entity reference target settings

Problem/Motivation

Allow specifying the target reference settings for an entity reference template field. Some fields may already specify some of these settings (log.location) but others will not. Eg: log.asset could be narrowed down to only allow selecting Plant assets.

The challenge is providing multiple entity reference selection handlers. These could then be an option in the widget settings page. Perhaps this module could ship with some simple defaults for each asset bundle?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes