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
client.setVerifier((action, request, listener) -> verifyComponentTemplateInstalled(calledTimes, action, request, listener));
        registry.clusterChanged(event);
        assertBusy(() -> assertThat(calledTimes.get(), equalTo(registry.getComponentTemplateConfigs().size())));
    }
    public void testThatUnversionedOldTemplatesAreUpgraded() throws Exception {
        DiscoveryNode node = TestDiscoveryNode.create("node");
        DiscoveryNodes nodes = DiscoveryNodes.builder().localNodeId("node").masterNodeId("node").add(node).build();
        ClusterChangedEvent event = createClusterChangedEvent(
            Collections.singletonMap(StackTemplateRegistry.LOGS_SETTINGS_COMPONENT_TEMPLATE_NAME, null),
            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())));
    }
    public void testMissingNonRequiredTemplates() throws Exception {
        StackRegistryWithNonRequiredTemplates registryWithNonRequiredTemplate = new StackRegistryWithNonRequiredTemplates(
            Settings.EMPTY,
            clusterService,
            threadPool,
            client,
            NamedXContentRegistry.EMPTY
        );
        DiscoveryNode node = TestDiscoveryNode.create("node");
        DiscoveryNodes nodes = DiscoveryNodes.builder().localNodeId("node").masterNodeId("node").add(node).build();
        ClusterChangedEvent event = createClusterChangedEvent(
            Collections.singletonMap(StackTemplateRegistry.LOGS_SETTINGS_COMPONENT_TEMPLATE_NAME, null),
            nodes
        );