bigquery-storage: set gRPC keepalive by default on BigQueryReadClient #18784

ReadRows streams can sit idle for a long time, for example while BigQuery finishes a query job before the first row arrives. Without keepalive pings, a NAT or load balancer can drop the idle connection without telling either side.

google-cloud-bigquery works around this for Arrow query results (#18652) by building its own gRPC transport with grpc.keepalive_time_ms and grpc.keepalive_timeout_ms (_create_bqstorage_transport). As suggested in review on #18652, this belongs in google-cloud-bigquery-storage, so every long ReadRows read gets it.

Proposal: when the caller doesn't pass a transport or channel, BigQueryReadClient adds keepalive options to its default gRPC channel.

Open question: this changes the default channel for all bigquery-storage users, so the values and the default need agreement from the bigquery-storage owners.

Once this is released and google-cloud-bigquery requires it, google-cloud-bigquery can drop _create_bqstorage_transport (tracked in #18785).