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
DiscoveryNode localNode = TestDiscoveryNode.create("node");
        DiscoveryNodes nodes = DiscoveryNodes.builder().localNodeId("node").add(localNode).build();
        client.setVerifier((a, r, l) -> {
            fail("if the master is missing nothing should happen");
            return null;
        });
        ClusterChangedEvent event = createClusterChangedEvent(
            Collections.singletonMap(StackTemplateRegistry.LOGS_INDEX_TEMPLATE_NAME, null),
            nodes
        );
        registry.clusterChanged(event);
    }
    
    /**
     * A client that delegates to a verifying function for action/request/listener
     */
    public static class VerifyingClient extends NoOpClient {
        private TriFunction<ActionType<?>, ActionRequest, ActionListener<?>, ActionResponse> verifier = (a, r, l) -> {
            fail("verifier not set");
            return null;
        };
        VerifyingClient(ThreadPool threadPool) {
            super(threadPool);
        }
        @Override
        @SuppressWarnings("unchecked")
        protected <Request extends ActionRequest, Response extends ActionResponse> void doExecute(
            ActionType<Response> action,
            Request request,
            ActionListener<Response> listener