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
) {
            try {
                listener.onResponse((Response) verifier.apply(action, request, listener));
            } catch (Exception e) {
                listener.onFailure(e);
            }
        }
        public VerifyingClient setVerifier(TriFunction<ActionType<?>, ActionRequest, ActionListener<?>, ActionResponse> verifier) {
            this.verifier = verifier;
            return this;
        }
    }
    private ActionResponse verifyComponentTemplateInstalled(
        AtomicInteger calledTimes,
        ActionType<?> action,
        ActionRequest request,
        ActionListener<?> listener
    ) {
        if (action instanceof PutComponentTemplateAction) {
            calledTimes.incrementAndGet();
            assertThat(action, instanceOf(PutComponentTemplateAction.class));
            assertThat(request, instanceOf(PutComponentTemplateAction.Request.class));
            final PutComponentTemplateAction.Request putRequest = (PutComponentTemplateAction.Request) request;
            assertThat(putRequest.componentTemplate().version(), equalTo((long) StackTemplateRegistry.REGISTRY_VERSION));
            assertNotNull(listener);
            return new TestPutIndexTemplateResponse(true);
        } else if (action instanceof PutLifecycleAction) {
            
            return AcknowledgedResponse.TRUE;
        } else if (action instanceof PutComposableIndexTemplateAction) {