Add SSERouter

[m110]

This is a proposal for a thin abstraction of SSE router and handlers.

The idea is that it's enough to prepare two method for implementing a SSE handler:

• GetResponse - returns a model of response to be marshalled and rendered. Typically, this will get some kind of identifier from request (r) and fetch the payload from a repository / application query handler.
• Validate - checks if the incoming message should be handled. For example, the handler could consume post-updated topic, which has updates to all posts in the system. This method should unmarshall the payload and check if the ID matches the one in request (r) - or even better, check in message's metadata if available.

See tests for an example implementation.

How the handler works:

• If there's no event-stream in Accept header, return model from ResponseProvider.
• Otherwise, return model from GetResponse and start the stream.
• Check each incoming Message with Validate and push new model to stream each time it passes.

Out of scope / TODO:

• Tests
• Better interface and method names
• Godoc
• Consider adding "ping" functionality to keep the connection alive.
  • chi seems to not support this. Maybe in future we can add our own responder.
• Will add a full example in separate PR to the main repository.

[maclav3]

Maybe we could add some documentation about what to pass as upstreamRouter here. upstreamSubscriber is maybe a little bit more intuitive, but what is the router here for?
What arguments to pass to the constructor?

I figure that there will be an example for SSE in due time, but I feel that a godoc might help

[m110]

Yeah, will add it. 👍