Support server push in the webserver API

[romain-grecourt]

The webserver API needs to be updated in order to support server side push.
Could be supported by HTTP2 or websocket (which we don't have at the moment)

[dansiviter]

This would be needed for MP 2.0 support as SSE is part of the JAX-RS 2.1 spec.

[spericas]

We should already have SSE support via Jersey. We do have support for JAX-RS 2.1. The notion of server push is broad. E.g., in HTTP/2 it refers to the ability of send related resources ahead of time, before they are requested.

[AshwinPrabhuB]

WebSocket would be a nice addition for server pushed notifications that isn't the same as HTTP/2 which is more of a network optimization for responses at the protocol layer.

[spericas]

SSE support would likely require adding a new dependency to your project. For more information, see Jersey's documentation here https://jersey.github.io/documentation/latest/sse.html

[spericas]

I'm closing this issue because the discussion has diverged into multiple topics that are not all related. For information about WebSockets in Helidon, see #1231. SSE should already be supported via Jersey. Finally, I will create another issue that is specific to HTTP/2 server push of resources.