BigQuery Data Transfer: add synth metadata.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Cloning googleapis.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:90f9d15e9bad675aeecd586725bce48f5667ffe7d5fc4d1e96d51ff34304815b
Status: Image is up to date for googleapis/artman:latest
synthtool > Running generator for google/cloud/bigquery/datatransfer/artman_bigquerydatatransfer.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/bigquerydatatransfer-v1.
synthtool > Replaced 'from google.cloud.bigquery.datatransfer_v1.proto' in google/cloud/bigquery_datatransfer_v1/proto/datatransfer_pb2.py.
synthtool > Replaced 'from google.cloud.bigquery.datatransfer_v1.proto' in google/cloud/bigquery_datatransfer_v1/proto/datatransfer_pb2_grpc.py.
synthtool > Replaced 'google-cloud-bigquerydatatransfer' in google/cloud/bigquery_datatransfer_v1/gapic/data_transfer_service_client.py.
synthtool > Replaced 'import google.api_core.gapic_v1.method\n' in google/cloud/bigquery_datatransfer_v1/gapic/data_transfer_service_client.py.
synthtool > Cleaned up 0 temporary directories.
synthtool > Wrote metadata to synth.metadata.