Add a heartbeat executor for SSE emitters

[reda-alaoui]

Fix #33355

[rstoyanchev]

Thanks for putting together a pull request. Some comments below.

I don't think there needs to be a public SseEmitterHeartbeatExecutor contract and implementation. This could be simply a TaskScheduler with the rest managed internally in a dedicated package private class (similar to ReactiveTypeHandler).

Heartbeats should not be emitted if emitters are already busy sending. Perhaps each emitter can keep a timestamp of the last send, and expose a method to send a heartbeat that would send only if a certain amount of time has passed since the last send.

Controllers that use Flux for SSE can easily incorporate heartbeats. Such Flux return values are also turned into SseEmitters, and so adding heartbeats to every emitter you could end up with duplicate heartbeat streams for the same emitter. It makes me thing that emitters should actually opt into heartbeats, and then they should be called to send a heartbeat only if they opted in.