Add ID to Label schema

[tholor]

If we add new labels to elasticsearch and update_existing_documents=True for the DocumentStore, we will use elastic's _op_type create instead of index. However, this one requires us to supply an ID. Otherwise it crashes with:

elasticsearch.exceptions.RequestError: RequestError(400, 'action_request_validation_exception', 'Validation Failed: 1: an id must be provided if version type or value are set;')

This PR adds an ID field to the Label class in a similar way as we did with Document