Feature request

Add in an API to do asynchronous service callbacks for service servers.

Feature description

It is currently possible in ROS2 to have a service callback in a thread by using separate callback groups and using a multi-threaded executor. However, it would probably be nice to allow for an API where the user can specify that their service is asynchronous. During the initial call to the service, the service would kick off a thread (or whatever else is needed to do the work), then return some kind of promise. When the service eventually completes, it would signal the client that it has completed the promise. I'm not entirely sure what the API for this would look like currently, but @wjwwood may have some ideas here.