Decouple the background service from the actual attach.

[CZEMacLeod]

At the moment, the attach is done in the ResourceNotificationService WatchAsync enumerator.

We could have an event using the eventing system which tries to do the actual DebugAttachAsync. There is already code which creates a DebugAttachExecutionContext which could be used.

This would mean that when we have Skip enabled, (such as for Node) and are waiting for the debugger attach information, we could just push the same event.