Provide shutdown notification for scripts

[niloc132]

As a script author, I want to know when the server is shutting down so my scripts can finish work they are doing. For example, a script might write accumulated table contents to disk, and on shutdown any remaining contents should be written if possible.

It is understood that shutdowns cannot always be handled gracefully, but for "clean" shutdowns, it should be possible to give such a listener a chance to observe this happening and react.

The JVM provides the ability to add a shutdown task, by creating a new java.lang.Thread. Internally, deephaven-core has an API to handle shutdown, but it isn't part of the public API.

[rcaudy]

I assume we want to integrate this with application mode, and provide Java/Groovy and Python options.