[FEATURE] Gracefully shutdown function on SIGINT & SIGTERM

[webup]

Please describe your usage scenario

OpenFunction/functions-framework#22

When the function rollout or scaledown, we need the function to be gracefully shutdown to avoid losing the request or event.

Describe the solution you'd like

Describe alternatives you've considered

Additional context

node.js - How do I shut down my Express server gracefully when its process is killed? - Stack Overflow might be a good reference for the implementation.