Use on all fieldable content entity types

Problem/Motivation

The provided field is available to add to content entity types other than node and taxonomy_term, however, it does not work on those content entity types. For example, Media entities.

Steps to reproduce

Install this module and Media module.
Edit a Media type and add the Allow Only One field, configured as desired.
Create a media entity of that type and save it.
Create a second media entity of that type, with the same data in the restricted fields, and save it.
Observe that both media entities have been created, without validation error.

Proposed resolution

Patch the constraint validator to work with all fieldable content entity types.

Remaining tasks

User interface changes

API changes

Data model changes