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
StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)
        );
        versions.put(
            StackTemplateRegistry.LOGS_SETTINGS_COMPONENT_TEMPLATE_NAME,
            StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)
        );
        versions.put(
            StackTemplateRegistry.LOGS_MAPPINGS_COMPONENT_TEMPLATE_NAME,
            StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)
        );
        versions.put(
            StackTemplateRegistry.METRICS_SETTINGS_COMPONENT_TEMPLATE_NAME,
            StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)
        );
        versions.put(
            StackTemplateRegistry.METRICS_MAPPINGS_COMPONENT_TEMPLATE_NAME,
            StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)
        );
        versions.put(
            StackTemplateRegistry.SYNTHETICS_SETTINGS_COMPONENT_TEMPLATE_NAME,
            StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)
        );
        versions.put(
            StackTemplateRegistry.SYNTHETICS_MAPPINGS_COMPONENT_TEMPLATE_NAME,
            StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)
        );
        ClusterChangedEvent higherVersionEvent = createClusterChangedEvent(versions, nodes);
        registry.clusterChanged(higherVersionEvent);
    }
    public void testThatMissingMasterNodeDoesNothing() {