improve: possibility to provide id for external resource

docs/content/en/docs/documentation/dependent-resource-and-workflows/dependent-resources.md

@@ -303,14 +303,15 @@ they should be reconciled.
 There might be cases, though, where it might be problematic to call the `desired` method several times (for example, because it is costly to do so),
 it is always possible to override this automated discrimination using several means (consider in this priority order):
 
-- Override the `targetSecondaryResourceID` method, if your `DependentResource` extends `KubernetesDependentResource`,
+- Override the [`targetSecondaryResourceID`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/dependent/kubernetes/KubernetesDependentResource.java#L282)
+  method, if your `DependentResource` extends `KubernetesDependentResource`,
   where it's very often possible to easily determine the `ResourceID` of the secondary resource. This would probably be
-  the easiest solution if you're working with Kubernetes resources.
-- Override the `selectTargetSecondaryResource` method, if your `DependentResource` extends `AbstractDependentResource`.
-  This should be relatively simple to override this method to optimize the matching to your needs. You can see an
-  example of such an implementation in
-  the [`ExternalWithStateDependentResource`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework/src/test/java/io/javaoperatorsdk/operator/dependent/externalstate/ExternalWithStateDependentResource.java)
-  class. 
+  the easiest solution if you're working with Kubernetes resources. Similarly, you can override the 
+  [`targetSecondaryResourceID`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/dependent/AbstractExternalDependentResource.java#L138)
+  for external resources. See below the related ID handling
+- If the approach above doesn't fit your needs, you can override the target resource selection mechanism by overriding
+  `selectTargetSecondaryResource` for both [`KubernetesDependentResource`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/dependent/kubernetes/KubernetesDependentResource.java#L282)
+  and [`AbstractExternalDependentResource`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/dependent/AbstractExternalDependentResource.java#L148). 
 - As last resort, you can implement your own `getSecondaryResource` method on your `DependentResource` implementation from scratch.
 
 ### Sharing an Event Source Between Dependent Resources

operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/dependent/AbstractExternalDependentResource.java

@@ -126,19 +126,34 @@ protected InformerEventSource getExternalStateEventSource() {
     return externalStateEventSource;
   }
 
+  /**
+   * Override this method to optimize and avoid computing the desired when selecting the target
+   * secondary resource. Return the target id.
+   *
+   * @see ExternalDependentIDProvider
+   * @param primary resource
+   * @param context of current reconciliation
+   * @return id of the target managed resource
+   */
+  protected Optional<Object> targetSecondaryResourceID(P primary, Context<P> context) {
+    R desired = desired(primary, context);
+    if (desired instanceof ExternalDependentIDProvider<?> desiredWithId) {
+      return Optional.of(desiredWithId.externalResourceId());
+    } else {
+      return Optional.empty();
+    }
+  }
+
   @Override
   protected Optional<R> selectTargetSecondaryResource(
       Set<R> secondaryResources, P primary, Context<P> context) {
-    R desired = desired(primary, context);
+    var optionalId = targetSecondaryResourceID(primary, context);
     List<R> targetResources;
-    if (desired instanceof ExternalDependentIDProvider<?> desiredWithId) {
+    if (optionalId.isPresent()) {
+      var id = optionalId.get();
       targetResources =
           secondaryResources.stream()
-              .filter(
-                  r ->
-                      ((ExternalDependentIDProvider<?>) r)
-                          .externalResourceId()
-                          .equals(desiredWithId.externalResourceId()))
+              .filter(r -> ((ExternalDependentIDProvider<?>) r).externalResourceId().equals(id))
               .toList();
     } else {
       throw new IllegalStateException(