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
new NamedXContentRegistry(
                            List.of(
                                new NamedXContentRegistry.Entry(
                                    LifecycleAction.class,
                                    new ParseField(DeleteAction.NAME),
                                    DeleteAction::parse
                                )
                            )
                        )
                    ),
                    policyStr
                )
        ) {
            LifecyclePolicy different = LifecyclePolicy.parse(parser, policies.get(0).getName());
            policyMap.put(policies.get(0).getName(), different);
            ClusterChangedEvent event = createClusterChangedEvent(Collections.emptyMap(), policyMap, nodes);
            registry.clusterChanged(event);
        }
    }
    public void testThatVersionedOldTemplatesAreUpgraded() 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,
                StackTemplateRegistry.REGISTRY_VERSION - 1
            ),
            nodes
        );
        AtomicInteger calledTimes = new AtomicInteger(0);