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
SYNTHETICS_MAPPINGS_COMPONENT_TEMPLATE_NAME,
                "/synthetics-mappings.json",
                REGISTRY_VERSION,
                TEMPLATE_VERSION_VARIABLE
            ),
            new IndexTemplateConfig(
                SYNTHETICS_SETTINGS_COMPONENT_TEMPLATE_NAME,
                "/synthetics-settings.json",
                REGISTRY_VERSION,
                TEMPLATE_VERSION_VARIABLE
            )
        )) {
            try {
                componentTemplates.put(
                    config.getTemplateName(),
                    ComponentTemplate.parse(JsonXContent.jsonXContent.createParser(XContentParserConfiguration.EMPTY, config.loadBytes()))
                );
            } catch (IOException e) {
                throw new AssertionError(e);
            }
        }
        COMPONENT_TEMPLATE_CONFIGS = Map.copyOf(componentTemplates);
    }
    @Override
    protected Map<String, ComponentTemplate> getComponentTemplateConfigs() {
        if (stackTemplateEnabled) {
            return COMPONENT_TEMPLATE_CONFIGS;
        } else {
            return Map.of();
        }