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/main/java/org/elasticsearch/xpack/stack/StackTemplateRegistry.java
public static final String METRICS_ILM_POLICY_NAME = "metrics";
    public static final String METRICS_INDEX_TEMPLATE_NAME = "metrics";
    
    
    
    public static final String SYNTHETICS_MAPPINGS_COMPONENT_TEMPLATE_NAME = "synthetics-mappings";
    public static final String SYNTHETICS_SETTINGS_COMPONENT_TEMPLATE_NAME = "synthetics-settings";
    public static final String SYNTHETICS_ILM_POLICY_NAME = "synthetics";
    public static final String SYNTHETICS_INDEX_TEMPLATE_NAME = "synthetics";
    public StackTemplateRegistry(
        Settings nodeSettings,
        ClusterService clusterService,
        ThreadPool threadPool,
        Client client,
        NamedXContentRegistry xContentRegistry
    ) {
        super(nodeSettings, clusterService, threadPool, client, xContentRegistry);
        this.clusterService = clusterService;
        this.stackTemplateEnabled = STACK_TEMPLATES_ENABLED.get(nodeSettings);
    }
    @Override
    public void initialize() {
        super.initialize();
        clusterService.getClusterSettings().addSettingsUpdateConsumer(STACK_TEMPLATES_ENABLED, this::updateEnabledSetting);
    }
    private void updateEnabledSetting(boolean newValue) {
        if (newValue) {
            this.stackTemplateEnabled = true;
        } else {
            logger.info(