add support for dynamic entity reference fields

This module doesn't support reference fields from https://www.drupal.org/project/dynamic_entity_reference → . These are of a different type, and allow multiple entity types to be referenced in a single field.

I see that the DependencyFieldMapGenerator service has been written to take some aspects of the field type from the service definition as parameters:

    arguments: ['@entity_field.manager', '@entity_type.manager', 'entity_reference', 'target_type']

  public function __construct(EntityFieldManagerInterface $entityFieldManager, EntityTypeManagerInterface $entityTypeManager, $fieldType, $targetEntitySettingsKey) {

However, that's not going to work, as the $targetEntitySettingsKey is assumed to be a string value, whereas for DER fields it's an array:

        $target_entity_type_id = $source_entity_storage_definitions[$field_name]->getSetting($this->targetEntitySettingsKey);

Also, EntityReferenceIntegrityEntityHandler is only designed to work with a single service that provides the field map.

What I think would work is that we convert DependencyFieldMapGenerator to be a service collector, and then define two new tagged services that it picks up:

- EntityReferenceFieldMapGenerator
- DynamicEntityReferenceFieldMapGenerator

Each would be responsible for a single field type.