gRPC "subscribe" method on the COS client

[zenyui]

Is your feature request related to a problem? Please describe.
It would be nice to have a "subscription" pattern for changes, similar to implementing a read side. This would allow applications to register as streaming consumers at runtime instead of in the COS configuration. This could be used to back cool patterns like serving changes over websockets to external API clients, etc.

Describe the solution you'd like

• add a subscribe client method to the COS service, perhaps one that allows consuming changes for a particular entity or all entities
• implement the subscribe method in COS server as a gRPC stream. probably use akka streams, and find a way to do this without too many connections to the journal (if db reads are needed at all)

Describe alternatives you've considered
n/a

Additional context

• akka has cool streaming "Hub" concepts that allow you to change the streaming graph after initialization https://doc.akka.io/docs/akka/current/stream/stream-dynamic.html. Need to research how we would do late registration and broadcast changes on a node to all gRPC streams.
• we should allow all nodes to answer gRPC stream subscribe requests, instead of a cluster singleton, etc
• let's first get comfortable with gRPC streaming in gRPC streaming read side interface #346 and do proper research with akka streams