Give an option to override the default number of shards in BQ streaming inserts

[damccorm]

As per #12485 there's no way to override it, though Java does have one: 
https://github.com/apache/beam/blob/master/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryOptions.java#L58
 
 
Let's add a good way to override this.

Imported from Jira BEAM-10680. Original Jira may contain additional context.
Reported by: pabloem.

[dannikay]

.take-issue