Call out to a missing `@custom` pipeline so that users can add custom processors to the default logs pipeline.

Open questions
* Can we make the custom pipeline specific for a dataset?
* Should this pipeline also be called by integrations?
* Align with https://github.com/elastic/kibana/issues/149484

Relates to https://github.com/elastic/elasticsearch/pull/95481

cc @joshdover @kpollich 
x-pack/plugin/stack/src/test/java/org/elasticsearch/xpack/stack/StackTemplateRegistryTests.java
return AcknowledgedResponse.TRUE;
        } else {
            fail("client called with unexpected request:" + request.toString());
            return null;
        }
    }
    private ClusterChangedEvent createClusterChangedEvent(Map<String, Integer> existingTemplates, DiscoveryNodes nodes) {
        return createClusterChangedEvent(existingTemplates, Collections.emptyMap(), nodes);
    }
    private ClusterChangedEvent createClusterChangedEvent(
        Map<String, Integer> existingTemplates,
        Map<String, LifecyclePolicy> existingPolicies,
        DiscoveryNodes nodes
    ) {
        ClusterState cs = createClusterState(Settings.EMPTY, existingTemplates, existingPolicies, nodes);
        ClusterChangedEvent realEvent = new ClusterChangedEvent(
            "created-from-test",
            cs,
            ClusterState.builder(new ClusterName("test")).build()
        );
        ClusterChangedEvent event = spy(realEvent);
        when(event.localNodeMaster()).thenReturn(nodes.isLocalNodeElectedMaster());
        return event;
    }
    private ClusterState createClusterState(
        Settings nodeSettings,
        Map<String, Integer> existingComponentTemplates,
        Map<String, LifecyclePolicy> existingPolicies,
        DiscoveryNodes nodes