We need a way to register handlers that can generate RDF for values of a given data type. Steps:
- Refactor StatementRdfBuilder to introduce a DataValueRdfBuilder interface
- Implement a dispatching DataValueRdfBuilder (or SnakRdfBuilder) that dispatches based on the data type
- Add getRdfBuilderFactoryCallbacks() to DataTypeDefinitions
- Register RDF handlers (mappings) for well known data types in WikibaseRepo.datatypes.php