Provide notification on call when transport is shut down

This can apply on server-side when the server as a whole is shutting down and also when MAX_CONNECTION_AGE is triggered for a particular transport. It would just be a notification to allow the application to perform a graceful call closure.

It'd also be useful on client-side, especially on mobile (#3688), when connections are being migrated.

There may be concerns of both the client and server reacting at the same time. It should also be noted that in some cases only one side may receive the notification (i.e., when a L7 load balancer is involved).

I don't think any language provides this today. If it is deemed useful, this should be a cross-language feature.