brat serializer: allow using annotation ids from document metadata

[ArneBinder]

This PR adds the parameters metadata_gold_id_keys and metadata_prediction_id_keys to the BratSerializer:

• metadata_gold_id_keys: A dictionary mapping layer names to metadata keys that contain the gold annotation IDs.
• metadata_prediction_id_keys: A dictionary mapping layer names to metadata keys that contain the prediction annotation IDs.

Usage example:

+serializer.metadata_gold_id_keys.labeled_spans=entity_ids 

This will use the entries in entity_ids from the document metadata as IDs when serializing the gold annotations from the labeled_spans layer.