Support `Flux<ServerSentEvent<Fragment>>` in WebFlux

[dsyer]

In Spring 6.2 we got a new WebFlux feature with Fragment and you can render an SSE like this:

@GetMapping(path = "/stream", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<Fragment> stream() {
	return Flux.interval(Duration.ofSeconds(5))
			.map(value -> Fragment.create("time", Map.of("value", value, "time", System.currentTimeMillis())));
}

The only potential problem is that it doen't give you control over the SSE fields (in particular the "event" name which is unconditionaly rendered as the view name). If we could send Flux<ServerSentEvent<Fragment>> it would give us more control.

Same for MVC, but I think that might already be supported (Flux<ServerSentEvent<ModelAndView>>)?

[rstoyanchev]

In Spring MVC, the equivalent is SseEmitter<ModelAndView> and that is already supported. It makes sense to add the same for WebFlux in order to allow control over SSE fields with Fragment rendering.