[Feature Request] Provide a typed result API

Is your feature request related to a problem? Please describe.

I would like to retrieve the result of a workflow run. The workflow method has a return type but this is not taken into account because getResult only exists on the untyped stub as the typed stub is a proxy class to the workflow and calls workflow class methods but the untyped stub doesn't have type info.

Describe the solution you'd like

There should be a higher level API than WorkflowRunInterface::getResult which can infer the type automatically from the return type of the workflow method. Since the return workflow method must be a supertype of Generator this might need the ability to handle composite types, there are multiple type libraries for example symfony/type-info although I have simply done

    $workflowReturnType = $this->temporalFactories
      ->getWorkflowStub($workflowClass)
      ->getHandlerReflection()
      ->getReturnType();
    if ($workflowReturnType instanceof \ReflectionUnionType &&
      ($types = array_diff(array_map('strval', $workflowReturnType->getTypes()), ['Generator']))) {
      return reset($types);
    }

in my workaround to the problem.

Start with WorkflowRunInterface::getResult and the workflow stub path through getWorkflowStub, getHandlerReflection, and getReturnType. Review how the Generator return type and union or composite types are represented, then define an API that infers the workflow result type automatically; done means typed workflow results can be retrieved without the workaround described.