Java BigQueryIO clustering support breaks on multiple partitions

Support for writing to clustered tables in BigQuery was added in 2.15, which involved adding a new optional clustering field to TableDestination. Clustering support is working for most cases, but fails with errors about incompatible partitioning specifications for any data that is handled by the MultiplePartitions branch of BigQueryIO logic.

There is a case in that code path where we provide a modified TableDestination and neglect to copy the clustering definition, so the final load job does not include any clustering columns.