Add a class for bundle fields definitions

The modules that want add fields to an entity bundle field in hook_entity_bundle_field_info() don't have a class to use for that. The BaseFieldDefinition class is the wrong one to use, since its isBaseField() returns TRUE, when they need it returns FALSE. They would end up with defining their own class just to override that method.

Drupal core should define that class for them, so third-party modules don't need to define that class.