Use Fastify hooks to shut down services gracefully

[timostamm]

Is your feature request related to a problem? Please describe.
When setting up a server, it is possible to pass a shutdownSignal:

  /**
   * To shut down servers gracefully, this option takes an AbortSignal.
   * If this signal is aborted, all signals in handler contexts will be aborted
   * as well. This gives implementations a chance to wrap up work before the
   * server process is killed.
   * Abort this signal with a ConnectError to send a message and code to
   * clients.
   */
  shutdownSignal?: AbortSignal;

This signal will also trigger the signal in the HandlerContext available to all RPC implementations, allowing them to close any open resources.

This is a convenient feature for microservices that might teared down frequently, but the feature is not well integrated into server plugins.

Describe the solution you'd like
Fastify has a close and preClose hooks that @connectrpc/connect-fastify could use to trigger the signal automatically.

Additional context
Usually, services are allowed a grace period to shut down cleanly, but it is unclear how the service could signal back that it is finished.