BigQueryUtils toBeamRow to support the bytes type

The BigQueryUtils toBeamRow implementation doesn't currently support the Avro/Beam bytes type via the convertAvroFormat method: https://github.com/apache/beam/blob/master/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryUtils.java#L558. A simple implementation which casts the Avro value to a ByteBuffer instance and returns the underlying byte array should suffice.