Expose activeInstances() accessor on WorkflowDefinition (#1678)

Signed-off-by: Matheus Cruz <matheuscruz.dev@gmail.com>

Author: mcruzdev

impl/core/src/main/java/io/serverlessworkflow/impl/WorkflowDefinition.java

@@ -196,6 +196,10 @@ public Optional<WorkflowInstance> activeInstance(String instanceId) {
     return Optional.ofNullable(activeInstances.get(instanceId));
   }
 
+  public Collection<WorkflowInstance> activeInstances() {
+    return Collections.unmodifiableCollection(activeInstances.values());
+  }
+
   @Override
   public WorkflowDefinitionId id() {
     return definitionId;