PoWeb: Subscribe to new parcel notifications with GCP PubSub

[gnarea]

Describe the problem

We currently use NATS Streaming in PoWeb's collectParcels method to be notified about new parcels, but we're deprecating NATS Streaming in production (relaycorp/cloud-gateway#84).

Describe the solution you'd like

This messaging channel should continue to be powered by NATS Streaming in development, but it should support GCP PubSub as well.

We might want to factor out this broker-agnostic functionality into an external library for non-durable subscriptions (@relaycorp/cloud-notifications?).

Pay a lot of attention to the quotas and limits of PubSub; in particular:

• Max subscriptions (10,000) per project and per topic. We might want to have one subscription per process, and use an EventEmitter to notify each WebSocket connection... Which would unfortunately mean that we could have fewer than 10k processes, unless we build a new internal service to fan out notifications.
• Publisher throughput per region (200 MB/s in small regions). So we shouldn't be sending too much data (we won't be the case once Store incoming, unprocessed cargo in object store #746 is fixed).

Describe alternatives you've considered

None

Additional context

This isn't the only use of NATS Streaming. See #759

[gnarea]

Superseded by #929.