Support entity types without bundles or without bundle config entities

[claudiu-cristea]

Cloned from ec-europa/rdf_entity#18

Right now, the module has a limitation: It works only with entity types that are using config entities in order to define bundles.

1. Support also:
   • Entities without bundles. The user entity is such an example.
   • Entities with bundles but without bundle entity types. Example: entity_test.
2. Integrate configurable fields in the rdf_entity_mapping entity. Now they are spread in the field storage configs. We may consider to add them in the bundle rdf_entity_mapping entity so we know that all mappings are stored in a single centralized place.
3. Unify all mappings from an entity type into a single mapping config, per entity type. this is debatable. We might want a config entity for entity type (holding base fields, etc) and switch back to bundle 3rd party. EDIT: We keep the actual model rdf_entity.mapping.{entity_type}.{bundle}.yml.