Creates an extractor for extracting features.
The extractor's PTransform extracts features from an Arrow RecordBatch stored
under tfma.ARROW_RECORD_BATCH_KEY in the incoming extract and adds them to the
output extract under the key tfma.FEATURES_KEY. Any extracts that already
exist will be merged with the values parsed by this extractor with this
extractor's values taking precedence when duplicate keys are detected.
Extractor for extracting features.