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 if (action instanceof PutComponentTemplateAction) {
                
                return new StackTemplateRegistryTests.TestPutIndexTemplateResponse(true);
            } else if (action instanceof PutComposableIndexTemplateAction) {
                
                return AcknowledgedResponse.TRUE;
            } else {
                fail("client called with unexpected request: " + request.toString());
                return null;
            }
        });
        ClusterChangedEvent event = createClusterChangedEvent(Collections.emptyMap(), nodes);
        registry.clusterChanged(event);
        assertBusy(() -> assertThat(calledTimes.get(), equalTo(8)));
    }
    public void testPolicyAlreadyExists() {
        DiscoveryNode node = TestDiscoveryNode.create("node");
        DiscoveryNodes nodes = DiscoveryNodes.builder().localNodeId("node").masterNodeId("node").add(node).build();
        Map<String, LifecyclePolicy> policyMap = new HashMap<>();
        List<LifecyclePolicy> policies = registry.getPolicyConfigs();
        assertThat(policies, hasSize(8));
        policies.forEach(p -> policyMap.put(p.getName(), p));
        client.setVerifier((action, request, listener) -> {
            if (action instanceof PutComponentTemplateAction) {
                
                return AcknowledgedResponse.TRUE;
            } else if (action instanceof PutLifecycleAction) {
                fail("if the policy already exists it should not be re-put");
            } else {