Allow using Kotlin Flow for SSE

[a-frank]

Description

At the moment in Quarkus it is possible to use Mutiny Multi with the annotation @RestSseElementType to create an endpoint sending events to the client as they are created.
Like so:

@RestSseElementType(MediaType.TEXT_PLAIN)
fun doSse(): Multi<String> = Multi.createFrom().
    emitter{ emitter ->
        emitter.emit("One")
        emitter.emit("Two")
        emitter.emit("Three")
        emitter.emit("Four")
        emitter.complete()
    }

Since Quarkus 2.0.0 it is possible to use Kotlin Coroutines with a suspend function for single request.

Support for Kotlin Flow for SSE would be a welcome addition.
Something like the following code snippet:

@RestSseElementType(MediaType.TEXT_PLAIN)
fun doFlow(): Flow<String> = flow{
    emit("One")
    emit("Two")
    emit("Three")
    emit("Four")
}

At the moment when using the above code the request returns immediately with a 200 but no values.

[quarkus-bot]

/cc @evanchooly

[geoand]

This would be a nice enhancement for RESTEasy Reactive.

Do you want to look into it @evanchooly or should I?

[eldimious]

+1

[geoand]

This part introduces support: #18222

It's a good start, but will certainly need more community testing when / if it gets in