Avoid addShutdownHook to JVM during its shutdown

[Lincong]

Discussion context.

Currently a JVM shutdown hook is added when DefaultClientFactory is used for the first time (code). It is possible that a JVM shutdown hook is being added after the JVM has started its shutdown and results in an exception below:

Caused by: java.lang.IllegalStateException: Shutdown in progress
    at java.base/java.lang.ApplicationShutdownHooks.add(ApplicationShutdownHooks.java:66)
    at java.base/java.lang.Runtime.addShutdownHook(Runtime.java:216)
    at grpc_shaded.com.linecorp.armeria.client.DefaultClientFactory.<clinit>(DefaultClientFactory.java:79)
    ... 24 more

More specifically, one situation can be that a client (e.g. Armeria gRPC client) was created for the first time leads to DefaultClientFactory being used for the first time. The JVM is already in shutdown when that happens.

A simple improvement could be:

static {
    if (DefaultClientFactory.class.getClassLoader() == ClassLoader.getSystemClassLoader()) {
      if (!shutdownHookDisabled) { // Skip adding the hook if we explicitly disabled it.
        Runtime.getRuntime().addShutdownHook(new Thread(() -> {
          ClientFactory.closeDefault();
        }));
      }
    }
}

and/or try-catch the java.lang.IllegalStateException: Shutdown in progress exception around Runtime.getRuntime().addShutdownHook and log something like "Trying to add shutdown hook during JVM shutdown, skip it because it does not matter any more since the JVM is already shutting down :)".

[thachlp]

Just create a pr as @Lincong suggest.
@ikhoon please help review