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
ClusterChangedEvent newEvent = createClusterChangedEvent(Collections.emptyMap(), nodes);
            registry.clusterChanged(newEvent);
            assertThat(calledTimes.get(), greaterThan(1));
        });
    }
    public void testThatNonExistingPoliciesAreAddedImmediately() throws Exception {
        DiscoveryNode node = TestDiscoveryNode.create("node");
        DiscoveryNodes nodes = DiscoveryNodes.builder().localNodeId("node").masterNodeId("node").add(node).build();
        AtomicInteger calledTimes = new AtomicInteger(0);
        client.setVerifier((action, request, listener) -> {
            if (action instanceof PutLifecycleAction) {
                calledTimes.incrementAndGet();
                assertThat(action, instanceOf(PutLifecycleAction.class));
                assertThat(request, instanceOf(PutLifecycleAction.Request.class));
                final PutLifecycleAction.Request putRequest = (PutLifecycleAction.Request) request;
                assertThat(
                    putRequest.getPolicy().getName(),
                    anyOf(
                        equalTo(StackTemplateRegistry.LOGS_ILM_POLICY_NAME),
                        equalTo(StackTemplateRegistry.METRICS_ILM_POLICY_NAME),
                        equalTo(StackTemplateRegistry.SYNTHETICS_ILM_POLICY_NAME),
                        equalTo(StackTemplateRegistry.ILM_7_DAYS_POLICY_NAME),
                        equalTo(StackTemplateRegistry.ILM_30_DAYS_POLICY_NAME),
                        equalTo(StackTemplateRegistry.ILM_90_DAYS_POLICY_NAME),
                        equalTo(StackTemplateRegistry.ILM_180_DAYS_POLICY_NAME),
                        equalTo(StackTemplateRegistry.ILM_365_DAYS_POLICY_NAME)
                    )
                );
                assertNotNull(listener);