Provide a way to terminate unfinished requests after graceful shutdown

[ikhoon]

Motivation:

Unfinished requests even after graceful shutdown period are forcivily closed with ClosedSessionException. As ClosedSessionException indicates that the connection was unexpectedly disconnected, ClosedSessionException is not suitable for graceful shutdown.

In this PR, I propose to add ShuttingDownException to terminate unfinished requests when a server stops.

Modifications:

• Introduce GracefulShutdown to customize graceful shutdown behavior.
  • Users can specify a error function to create an exception to unfinished terminate requests.
• Fixed HttpServerHandler to send error responses using the error function of GracefulShutdown
• Fixed Server to send error respones first and then close the connnections.
• Deprecation) ServerConfig.gracefulShutdownQuietPeriod() and ServerConfig.gracefulShutdownTimeout() have been deprecated in favor of ServerConfig.gracefulShutdown().

Result:

You can now use GracefulShutdown to terminate unfinished requests when a server stops.

GracefulShutdown gracefulShutdown =
  GracefulShutdown
    .builder()
    .quietPeriod(Duration.ofSeconds(10))
    .timeout(Duration.ofSeconds(15))
    .shutdownErrorFunction((ctx, req) -> {
        return new ServerStopException();
    })
    .build();

Server
  .builder()
  .gracefulShutdown(gracefulShutdown);

[github-actions[bot]]

🔍 Build Scan® (commit: f3f9b1f717fce5b6b0dae9049ec1de54b3662e3a)

Job name	Status	Build Scan®
build-ubicloud-standard-8-jdk-8	✅	https://ge.armeria.dev/s/ytcuyvikygk56
build-ubicloud-standard-8-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/aps7ijzua3p2g
build-ubicloud-standard-8-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/yeml4gvwm77ru
build-ubicloud-standard-8-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/voyhpybbh5sj4
build-ubicloud-standard-8-jdk-17-leak	✅	https://ge.armeria.dev/s/dynijahxidow2
build-ubicloud-standard-8-jdk-11	❌ (failure)	https://ge.armeria.dev/s/ldqtccbuke3sk
build-macos-latest-jdk-21	✅	https://ge.armeria.dev/s/ieamd53bc3tz6

[ikhoon]

Changed the target milestone to 1.32.0. I need some time to figure out how to implement the API that Trustin suggested.