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 Setting<Boolean> STACK_TEMPLATES_ENABLED = Setting.boolSetting(
        "stack.templates.enabled",
        true,
        Setting.Property.NodeScope,
        Setting.Property.Dynamic
    );
    private final ClusterService clusterService;
    private volatile boolean stackTemplateEnabled;
    
    public static final String DATA_STREAMS_MAPPINGS_COMPONENT_TEMPLATE_NAME = "data-streams-mappings";
    
    
    
    public static final String ILM_7_DAYS_POLICY_NAME = "7-days-default";
    public static final String ILM_30_DAYS_POLICY_NAME = "30-days-default";
    public static final String ILM_90_DAYS_POLICY_NAME = "90-days-default";
    public static final String ILM_180_DAYS_POLICY_NAME = "180-days-default";
    public static final String ILM_365_DAYS_POLICY_NAME = "365-days-default";
    
    
    
    public static final String LOGS_MAPPINGS_COMPONENT_TEMPLATE_NAME = "logs-mappings";
    public static final String LOGS_SETTINGS_COMPONENT_TEMPLATE_NAME = "logs-settings";
    public static final String LOGS_ILM_POLICY_NAME = "logs";
    public static final String LOGS_INDEX_TEMPLATE_NAME = "logs";
    
    
    
    public static final String METRICS_MAPPINGS_COMPONENT_TEMPLATE_NAME = "metrics-mappings";
    public static final String METRICS_SETTINGS_COMPONENT_TEMPLATE_NAME = "metrics-settings";