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
"/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();
        }
    }
    private static final Map<String, ComposableIndexTemplate> COMPOSABLE_INDEX_TEMPLATE_CONFIGS = parseComposableTemplates(
        new IndexTemplateConfig(LOGS_INDEX_TEMPLATE_NAME, "/logs-template.json", REGISTRY_VERSION, TEMPLATE_VERSION_VARIABLE),
        new IndexTemplateConfig(METRICS_INDEX_TEMPLATE_NAME, "/metrics-template.json", REGISTRY_VERSION, TEMPLATE_VERSION_VARIABLE),
        new IndexTemplateConfig(SYNTHETICS_INDEX_TEMPLATE_NAME, "/synthetics-template.json", REGISTRY_VERSION, TEMPLATE_VERSION_VARIABLE)
    );
    @Override