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
}
    private static final Map<String, ComposableIndexTemplate> COMPOSABLE_INDEX_TEMPLATE_CONFIGS = parseComposableTemplates(
        new IndexTemplateConfig(LOGS_INDEX_TEMPLATE_NAME, "/logs-template.json", REGISTRY_VERSION, TEMPLATE_VERSION_VARIABLE),
        new IndexTemplateConfig(METRICS_INDEX_TEMPLATE_NAME, "/metrics-template.json", REGISTRY_VERSION, TEMPLATE_VERSION_VARIABLE),
        new IndexTemplateConfig(SYNTHETICS_INDEX_TEMPLATE_NAME, "/synthetics-template.json", REGISTRY_VERSION, TEMPLATE_VERSION_VARIABLE)
    );
    @Override
    protected Map<String, ComposableIndexTemplate> getComposableTemplateConfigs() {
        if (stackTemplateEnabled) {
            return COMPOSABLE_INDEX_TEMPLATE_CONFIGS;
        } else {
            return Map.of();
        }
    }
    @Override
    protected String getOrigin() {
        return ClientHelper.STACK_ORIGIN;
    }
    @Override
    protected boolean requiresMasterNode() {
        
        
        
        
        
        
        return true;
    }
}