BigQueryUtils DATETIME conversion requires unix millis

I'm trying to run a very simple GCP Dataflow SQL job (copy all columns from table A to B), and it's failing with the following exception:
```

Caused by: java.lang.ClassCastException: org.apache.avro.util.Utf8 cannot be cast to java.lang.Longat org.apache.beam.sdk.io.gcp.bigquery.BigQueryUtils.truncateToMillis (BigQueryUtils.java:643)at org.apache.beam.sdk.io.gcp.bigquery.BigQueryUtils.convertAvroFormat (BigQueryUtils.java:597)at org.apache.beam.sdk.io.gcp.bigquery.BigQueryUtils.lambda$toBeamRow$5 (BigQueryUtils.java:389)

```

It looks to me like this is because BigQueryUtils.convertAvroFormat expects that any value whose (beam) type is "DATETIME" must be expressed in unix millis. However, these values come out of BigQuery as a string, (AFAICT) in ISO 8601 format.

Imported from Jira [BEAM-10021](https://issues.apache.org/jira/browse/BEAM-10021). Original Jira may contain additional context.
Reported by: benbirt.