Shutdown gracefully on SIGTERM or SIGINT

[dlvenable]

Is your feature request related to a problem? Please describe.

When Data Prepper receives a SIGTERM or SIGINT (ctrl-C), Data Prepper should begin a graceful shutdown.

Describe the solution you'd like

Handle a SIGTERM or SIGINT by adding a Java shutdown hook.

getRuntime().addShutdownHook( ... thread ...);

This shutdown should be similar to the POST :4900/shutdown API.