Override entity reference validation constraint of core workspaces

Problem/Motivation

It’s currently not possible to add entities which are not publishable and "revisionable" inside workspaces without applying corresponding changes to an entity type.

This is caused by WorkspaceManager::isEntityTypeSupported() which is gets called from EntityReferenceSupportedNewEntitiesConstraintValidator::validate() .

It would be great to have a simpler way to allow certain entity types to be created inside workspaces. In the long run workspaces should also adopt a concept of EntityTypeIsIgnored instead of EntityTypeIsSupported.

Steps to reproduce

1. Install a module that provides a content entity which is either no publishable or revisionable
2. Switch to a non live workspace
3. Try to create an entity of the type mentioned in step 1

Proposed resolution

For now override the validation constraint and its corresponding validator plugin and assign them in hook_field_info_alter(), dropping the validation constraint set by core workspaces.