CompletionStageRxInvokerImpl should not block on a thread

In comments of ApacheHttpAsyncClient4Engine in RestEasy code, it reads:

Again, there must be no blocking IO inside InvocationCallback! (If you are wondering why not to allow blocking calls by wrapping InvocationCallbacks in extra threads: Because then the main advantage of async IO, less threading, is lost.)

I agree with the statement. Wrapping blocking calls in extra threads is not the best way.
However, in the same code base, I see CompletionStageRxInvokerImpl has the following logic:

   @Override
   public CompletionStage<Response> get()
   {
      if (executor == null)
      {
         return CompletableFuture.supplyAsync(() -> builder.get());
      }
      else
      {
         return CompletableFuture.supplyAsync(() -> builder.get(), executor);
      }

   }

`builder.get()` in the above code is blocking.

I believe the limitation comes from JAX-RS spec, because it passes `SyncInvoker` to `RxInvokerProvider`. Also, `async` methods returning `java.util.concurrent.Future` instead of `java.util.concurrent.CompletionStage` puts limitations.

I think the above problem can be worked around by using `InvocationCallback` to complete/fail a `CompletableFuture`.

Here is the PR which I think should provide more details: https://github.com/resteasy/Resteasy/pull/1950

I will very much appreciate if you can share your comments/feedback.