[FEATURE REQUEST] Pub/Sub event system

[LdDl]

Is your feature request related to a problem? Please describe.
I would like to see pub/sub event system via gRPC
What is event in this context? Well, it just information about detected objects in certain moments of time.
Currently I see next possible events:

• object has crossed virtual line;
• object has entered virtual polygon;
• object has left virtual polygon;

Describe the solution you'd like
We need to make minimum valuable pub/sub system for client-side.
So we need to:

• improve the way protobuf files are used.
• describe possible event (via interface+structs or just structs)?
• implement pub/sub system (it will be gRPC as a wrapping, and channels as a core)

Describe alternatives you've considered

Additional context
Summary workflow for clients will be like:

1. Client connects to software instance
2. Client subscribes to events (may be with some filters turned on)
3. ODaM sends events to every subscribed client.

Also it will be needed an example of client-side (may be even for different programming languages)