Add a way for a custom executor to add a shutdown hook

[ltreffenstaedt-celonis]

We have a custom nx executor which is running one or more dev services. When the run is killed (e.g. using ctrl+c), those services do not shut down correctly. There is no way to shut them down, because we can only do that asynchronously. Usually you could do this in node.js using a beforeExit hook, but that is circumvented when process.exit() is called explicitly (as nx does).
Therefore, it would be great for nx to add an option to register a shutdown callback in a custom executor, which should be able to run async code.