Add a shutdown API to enable graceful termination of AnyVar

[ehclark]

The Snowflake storage backend writes VRS object batches asynchronously. Untimely termination could therefore result in pending batches being discarded. Providing an API for graceful shutdown would allow for all pending batches to be written prior to termination. This API should be callable as a PreStop hook in Kubernetes: https://kubernetes.io/docs/concepts/containers/container-lifecycle-hooks/

See also #64

[ehclark]

Given the process model used by ASGI, it would be improper to place a shutdown API in the app itself. Startup/shutdown is the responsibility of the web server, such as uvicorn or gunicorn.

Instead, AnyVar app should perform a graceful shutdown of the storage integration when a shutdown is occurs. This can be implemented by lifespan events, which are already in use for startup.