BigQuery: expose numDmlAffectedRows in query results

[tswast]

See: https://cloud.google.com/bigquery/docs/reference/rest/v2/jobs/query#numDmlAffectedRows

It is marked as experimental. This is because DML statements are currently in beta for BigQuery.

This will be needed to correctly implement cursor.rowcount for DB-API support #2434

[lukesneeringer]

Hey Tim,
Has this come out of the experimental phase yet?

[tseaver]

DML Docs still show beta status.

[tswast]

DML is GA now (for non-partitioned tables), by the way.
https://cloud.google.com/bigquery/docs/reference/standard-sql/data-manipulation-language