feat: don't install a shutdown hook by default

Fixes #434

Author: lburgazzoli

Diff for: operator-framework-core/src/main/java/io/javaoperatorsdk/operator/Operator.java

@@ -33,7 +33,10 @@ public Operator(KubernetesClient k8sClient, ConfigurationService configurationSe
     this.lock = new Object();
     this.controllers = new ArrayList<>();
     this.started = false;
+  }
 
+  /** Adds a shutdown hook that automatically calls {@link #close()} when the app shuts down. */
+  public void installShutdownHook() {
     Runtime.getRuntime().addShutdownHook(new Thread(this::close));
   }