User Defined Source: support for clean shutdown hook

[nagarajatantry]

In the User-defined source, I would like to have the ability to do a clean shutdown of the resources on Shutdown/Sigterm events. In the current interface, I do not see a way to do it. https://github.com/numaproj/numaflow-java/blob/main/src/main/java/io/numaproj/numaflow/sourcer/Sourcer.java