BigQueryUtils toBeamRow to support GenericRecords with millisecond precision timestamps

Currently, BigQueryUtils assumes all Avro GenericRecords which utilize timestamp fields include timestamp data in microseconds when converting from GenericRecords to Beam Rows (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#L568). The implementation should also support timestamp data in milliseconds; this should be feasible by referencing the LogicalType associated with the avro field to determine if the underlying value is represented in millis or micros.