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/test/java/org/elasticsearch/xpack/stack/StackTemplateRegistryTests.java
return AcknowledgedResponse.TRUE;
            } else if (action instanceof PutComposableIndexTemplateAction) {
                
                return AcknowledgedResponse.TRUE;
            } else {
                fail("client called with unexpected request:" + request.toString());
                return null;
            }
        });
        registry.clusterChanged(sameVersionEvent);
        versions.clear();
        versions.put(
            StackTemplateRegistry.DATA_STREAMS_MAPPINGS_COMPONENT_TEMPLATE_NAME,
            StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)
        );
        versions.put(
            StackTemplateRegistry.LOGS_SETTINGS_COMPONENT_TEMPLATE_NAME,
            StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)
        );
        versions.put(
            StackTemplateRegistry.LOGS_MAPPINGS_COMPONENT_TEMPLATE_NAME,
            StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)
        );
        versions.put(
            StackTemplateRegistry.METRICS_SETTINGS_COMPONENT_TEMPLATE_NAME,
            StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)
        );
        versions.put(
            StackTemplateRegistry.METRICS_MAPPINGS_COMPONENT_TEMPLATE_NAME,
            StackTemplateRegistry.REGISTRY_VERSION + randomIntBetween(1, 1000)