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
}
    /**
     * Retrieves a list of {@link IngestPipelineConfig} that represents the ingest pipelines
     * that should be installed and managed.
     * @return The configurations for ingest pipelines that should be installed.
     */
    protected List<IngestPipelineConfig> getIngestPipelines() {
        return Collections.emptyList();
    }
    /**
     * Retrieves an identifier that is used to identify which plugin is asking for this.
     * @return A string ID for the plugin managing these templates.
     */
    protected abstract String getOrigin();
    /**
     * Called when creation of an index template fails.
     * @param templateName the template name that failed to be created.
     * @param e The exception that caused the failure.
     */
    protected void onPutTemplateFailure(String templateName, Exception e) {
        logger.error(() -> format("error adding index template [%s] for [%s]", templateName, getOrigin()), e);
    }
    /**
     * Called when creation of a lifecycle policy fails.
     * @param policy The lifecycle policy that failed to be created.
     * @param e The exception that caused the failure.
     */
    protected void onPutPolicyFailure(LifecyclePolicy policy, Exception e) {
        logger.error(() -> format("error adding lifecycle policy [%s] for [%s]", policy.getName(), getOrigin()), e);
    }