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
) {
        Map<String, ComponentTemplate> componentTemplates = new HashMap<>();
        for (Map.Entry<String, Integer> template : existingComponentTemplates.entrySet()) {
            ComponentTemplate mockTemplate = mock(ComponentTemplate.class);
            when(mockTemplate.version()).thenReturn(template.getValue() == null ? null : (long) template.getValue());
            componentTemplates.put(template.getKey(), mockTemplate);
        }
        Map<String, LifecyclePolicyMetadata> existingILMMeta = existingPolicies.entrySet()
            .stream()
            .collect(Collectors.toMap(Map.Entry::getKey, e -> new LifecyclePolicyMetadata(e.getValue(), Collections.emptyMap(), 1, 1)));
        IndexLifecycleMetadata ilmMeta = new IndexLifecycleMetadata(existingILMMeta, OperationMode.RUNNING);
        return ClusterState.builder(new ClusterName("test"))
            .metadata(
                Metadata.builder()
                    .componentTemplates(componentTemplates)
                    .transientSettings(nodeSettings)
                    .putCustom(IndexLifecycleMetadata.TYPE, ilmMeta)
                    .build()
            )
            .blocks(new ClusterBlocks.Builder().build())
            .nodes(nodes)
            .build();
    }
    private static class TestPutIndexTemplateResponse extends AcknowledgedResponse {
        TestPutIndexTemplateResponse(boolean acknowledged) {
            super(acknowledged);
        }
    }
}