Add toCompletableFuture for CompletableDeferredObject

[khoatruongdang]

We need Promise4j can work with other Async Processing Framework, at least with CompletableFuture of Java 8 core, so we must add toCompletableFuture for CompletableDeferredObject

[dungba88]

Here is one simple way to do it:

public static <T, E> CompletableFuture toCompletableFuture(Promise<T, E> promise) {
    var future = new CompletableFuture<T>();
    promise.done(future::complete).fail(future::completeExceptionally);
    return future;
}

[khoatruongdang]

Thank for your help!!
I'm creating PR to add method in CompletableDeferredObject also:

public CompletableFuture<D> toCompletableFuture() {
        return future;
}