Support Flow or Channel as Subscription return type

When using Kotlin Coroutines in my project, I want to use Kotlin Coroutines API anywhere, it make my codes looks consistent, but it seems the Subscription does not support it.

```kotlin
val flow = MutableSharedFlow<Event>()

fun emitEvent() {
   flow.emit(event)
   ...
}

fun eventAdded()=flow
```
