feat(engine): activate multi instance children in batches

Instead of writing all the ACTIVATE commands for all children at once, we now use the
ProcessInstanceBatch.ACTIVATE command instead.

This command takes an index, which in this case is the amount of children that an ACTIVATE command will be written for.

engine/src/main/java/io/camunda/zeebe/engine/processing/bpmn/behavior/BpmnStateTransitionBehavior.java

@@ -304,6 +304,23 @@ public long activateChildInstanceWithKey(
     return childInstanceKey;
   }
 
+  /**
+   * Activate a given amount of children of a multi-instance element.
+   *
+   * @param context the context of the multi-instance element
+   * @param amount the amount of children for which we will write an activate command
+   */
+  public void activateChildInstancesInBatches(final BpmnElementContext context, final int amount) {
+    final var record =
+        new ProcessInstanceBatchRecord()
+            .setProcessInstanceKey(context.getProcessInstanceKey())
+            .setBatchElementInstanceKey(context.getElementInstanceKey())
+            .setIndex(amount);
+
+    final var key = keyGenerator.nextKey();
+    commandWriter.appendFollowUpCommand(key, ProcessInstanceBatchIntent.ACTIVATE, record);
+  }
+
   public void activateElementInstanceInFlowScope(
       final BpmnElementContext context, final ExecutableFlowElement element) {
 

engine/src/main/java/io/camunda/zeebe/engine/processing/bpmn/container/MultiInstanceBodyProcessor.java

@@ -274,7 +274,7 @@ private void activate(
     if (loopCharacteristics.isSequential()) {
       createInnerInstance(element, activated);
     } else {
-      inputCollection.forEach(item -> createInnerInstance(element, activated));
+      stateTransitionBehavior.activateChildInstancesInBatches(context, inputCollection.size());
     }
   }