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.

No se nombran archivos fuente ni pruebas. Empieza revisando el cierre del transporte de Java y las rutas relacionadas con la antigüedad de la conexión; después compara la gestión del ciclo de vida del cliente y del servidor, así como el issue móvil relacionado #3688. Para darlo por terminado, se necesita un contrato de notificación definido que tenga en cuenta el cierre simultáneo, la entrega en un solo sentido y la compatibilidad entre lenguajes.

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