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/core/src/main/java/org/elasticsearch/xpack/core/template/IndexTemplateRegistry.java
* This is useful for plugins where certain actions are performed on master nodes
     * and the templates should match the respective version.
     */
    protected boolean requiresMasterNode() {
        return false;
    }
    private void addTemplatesIfMissing(ClusterState state) {
        addLegacyTemplatesIfMissing(state);
        addComponentTemplatesIfMissing(state);
        addComposableTemplatesIfMissing(state);
    }
    private void addLegacyTemplatesIfMissing(ClusterState state) {
        final List<IndexTemplateConfig> indexTemplates = getLegacyTemplateConfigs();
        for (IndexTemplateConfig newTemplate : indexTemplates) {
            final String templateName = newTemplate.getTemplateName();
            final AtomicBoolean creationCheck = templateCreationsInProgress.computeIfAbsent(templateName, key -> new AtomicBoolean(false));
            if (creationCheck.compareAndSet(false, true)) {
                IndexTemplateMetadata currentTemplate = state.metadata().getTemplates().get(templateName);
                if (Objects.isNull(currentTemplate)) {
                    logger.debug("adding legacy template [{}] for [{}], because it doesn't exist", templateName, getOrigin());
                    putLegacyTemplate(newTemplate, creationCheck);
                } else if (Objects.isNull(currentTemplate.getVersion()) || newTemplate.getVersion() > currentTemplate.getVersion()) {
                    
                    
                    logger.info(
                        "upgrading legacy template [{}] for [{}] from version [{}] to version [{}]",
                        templateName,
                        getOrigin(),
                        currentTemplate.getVersion(),
                        newTemplate.getVersion()