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
super.tearDown();
        threadPool.shutdownNow();
    }
    public void testDisabledDoesNotAddTemplates() {
        Settings settings = Settings.builder().put(StackTemplateRegistry.STACK_TEMPLATES_ENABLED.getKey(), false).build();
        StackTemplateRegistry disabledRegistry = new StackTemplateRegistry(
            settings,
            clusterService,
            threadPool,
            client,
            NamedXContentRegistry.EMPTY
        );
        assertThat(disabledRegistry.getComponentTemplateConfigs(), anEmptyMap());
        assertThat(disabledRegistry.getComposableTemplateConfigs(), anEmptyMap());
        assertThat(disabledRegistry.getPolicyConfigs(), hasSize(0));
    }
    public void testThatNonExistingTemplatesAreAddedImmediately() throws Exception {
        DiscoveryNode node = TestDiscoveryNode.create("node");
        DiscoveryNodes nodes = DiscoveryNodes.builder().localNodeId("node").masterNodeId("node").add(node).build();
        ClusterChangedEvent event = createClusterChangedEvent(Collections.emptyMap(), nodes);
        AtomicInteger calledTimes = new AtomicInteger(0);
        client.setVerifier((action, request, listener) -> verifyComponentTemplateInstalled(calledTimes, action, request, listener));
        registry.clusterChanged(event);
        assertBusy(() -> assertThat(calledTimes.get(), equalTo(registry.getComponentTemplateConfigs().size())));
        calledTimes.set(0);
        
        
        
        
        assertBusy(() -> {