[feature] Hoping that `reduce` and `compensate` methods in Saga can support RxJava Observable/CompleteableFuture/Kotlin suspend functions

## Why you need it?

In Vertx or Spring Reacvice Database, reactive framework like RxJava or Reactor are wildly used. So hoping that Saga can support Reactive Programming

## The expected result

When define a saga action, I can use the following syntex:

```java
public interface InventoryAction {

// RxJava
Single reduce(String businessKey, int count);

// Java CompleteableFuture
CompleteableFuture compensateReduce(String businessKey);
}
```

or

```kotlin
interface InventoryAction {
// Kotlin coroutine, which can be mapped into RxJava or Java CompleteableFuture so it is not the primary goal
suspend fun reduce(val foo: String, val bar: Int): Boolean
}
```