weather-mv: Support streaming conversions to BigQuery.

[alxmrs]

Use Case

As a user, I want to be able to load real-time forecasts from cloud storage to BigQuery. I want to do this in the simplest, most efficient computational means. For example, ECMWF disseminates data 4x per day into cloud storage buckets. Once an upload event occurs, I want data to automatically start ingesting into BigQuery.

Proposal

Support a new argument --topic that lets users specify a GCP PubSub topic of CREATE or FINALIZE events to a GCS bucket. This argument or --uris will be required flags. Usage of --topic will deploy a streaming dataflow job that subscribes to PubSub events, windows them into 1 minute buckets, and performs the extract_rows operation on incoming data.

Note: for other clouds, users can bridge other notification systems to GCP's PubSub in the short term. Apache Kafka support can come later.