Add conversion between RxJava Completable and ListenableFuture<Void>

[TheKeveloper]

Currently, we can only convert from Single<T> to ListenableFuture<T>. We can't convert from RxJava Completable to Single<Void> because the only valid type for a Void is null, and RxJava explicitly prevents having a Single with a null value.

There is a way to workaround this by just using a dummy object as the value of the Single,

Futures.transform(
  FutureConverter.fromSingle(
    completable.toSingleDefault(new Object())), 
    _ignored -> null, 
    MoreExecutors.directExecutor())

But the above is pretty messy. It seems like it should be pretty straightforward to convert from Completable to ListenableFuture<Void> (or CompletableFuture<Void> for that matter).

Happy to take this on myself if this sounds reasonable!