Enhancing the `logs-*-*` index template with a default ingest pipeline that is first doing a pre-flight check if the `message` field might be JSON and then uses the JSON processor to decode the JSON and merge it top-level with the document.

See also this prototype: https://gist.github.com/felixbarny/a9a2f6243153d5508643fd95ac968a88#file-routing-yml-L114-L174

Open questions and things to consider
* How do users opt-out of default JSON parsing? They could override the index template. Maybe that's good enough.
* Similarly to the logs@custom component template, should we call out to a custom index pipeline? Align with naming in Fleet.
x-pack/plugin/stack/src/main/java/org/elasticsearch/xpack/stack/StackTemplateRegistry.java
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";
    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";