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
"stack composable templates [{}] and component templates [{}] will not be installed or reinstalled",
                String.join(",", getComposableTemplateConfigs().keySet()),
                String.join(",", getComponentTemplateConfigs().keySet())
            );
            this.stackTemplateEnabled = false;
        }
    }
    private static final List<LifecyclePolicy> LIFECYCLE_POLICY_CONFIGS = Stream.of(
        new LifecyclePolicyConfig(LOGS_ILM_POLICY_NAME, "/logs-policy.json"),
        new LifecyclePolicyConfig(METRICS_ILM_POLICY_NAME, "/metrics-policy.json"),
        new LifecyclePolicyConfig(SYNTHETICS_ILM_POLICY_NAME, "/synthetics-policy.json"),
        new LifecyclePolicyConfig(ILM_7_DAYS_POLICY_NAME, "/" + ILM_7_DAYS_POLICY_NAME + ".json"),
        new LifecyclePolicyConfig(ILM_30_DAYS_POLICY_NAME, "/" + ILM_30_DAYS_POLICY_NAME + ".json"),
        new LifecyclePolicyConfig(ILM_90_DAYS_POLICY_NAME, "/" + ILM_90_DAYS_POLICY_NAME + ".json"),
        new LifecyclePolicyConfig(ILM_180_DAYS_POLICY_NAME, "/" + ILM_180_DAYS_POLICY_NAME + ".json"),
        new LifecyclePolicyConfig(ILM_365_DAYS_POLICY_NAME, "/" + ILM_365_DAYS_POLICY_NAME + ".json")
    ).map(config -> config.load(LifecyclePolicyConfig.DEFAULT_X_CONTENT_REGISTRY)).toList();
    @Override
    protected List<LifecyclePolicy> getPolicyConfigs() {
        if (stackTemplateEnabled) {
            return LIFECYCLE_POLICY_CONFIGS;
        } else {
            return Collections.emptyList();
        }
    }
    private static final Map<String, ComponentTemplate> COMPONENT_TEMPLATE_CONFIGS;
    static {
        final Map<String, ComponentTemplate> componentTemplates = new HashMap<>();
        for (IndexTemplateConfig config : List.of(
            new IndexTemplateConfig(