List config entities in field storage settings

Problem/Motivation

DER fields can currently (using version 2) reference drupal config entities using the UI as long as the possible selection (field settings) has not been set

• exclude the selected items
• none selected

As there is no implementation for config entities in the aforementioned field settings, choosing any other combination will remove the possibility of selecting a config entity.

The current state therefore is either:

• show ALL possible referable entity types (including config entities) OR
• list only non-config entity types

Proposed resolution

Add option / selection for config entity types to the field storage settings page (to match the regular entity reference field type). This actually is done by removing artificial restriction. Admins / users with the permission to change these settings should be considered mature enough to know what they are doing. Restricting options to numeric ID entities only, has no true advantage - but does hindere development.

See patch

Remaining tasks

reviews needed; further polish

User interface changes

See proposed solution

API changes

none

Data model changes

none