Support server sent events (SSE) with Reactive/Flow

[kdavisk6]

Similar to #19, Server Sent Events are a one-way push style execution mode that FeignX should consider supporting. This can be accomplished easily if we have support reactive streams, but we should also consider provided support that does not rely on reactive streams or Flow.

One possibility is to provide a specific ResponseDecoder that can process responses with the content type of text/event-stream and push all decoded responses to a java.util.Queue. I suspect that there are more interesting and clever ways of accomplishing this, but I think that having something user's can take advantage of out of the box would be useful.