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
protected final Settings settings;
    protected final Client client;
    protected final ThreadPool threadPool;
    protected final NamedXContentRegistry xContentRegistry;
    protected final ClusterService clusterService;
    protected final ConcurrentMap<String, AtomicBoolean> templateCreationsInProgress = new ConcurrentHashMap<>();
    protected final ConcurrentMap<String, AtomicBoolean> policyCreationsInProgress = new ConcurrentHashMap<>();
    protected final ConcurrentMap<String, AtomicBoolean> pipelineCreationsInProgress = new ConcurrentHashMap<>();
    public IndexTemplateRegistry(
        Settings nodeSettings,
        ClusterService clusterService,
        ThreadPool threadPool,
        Client client,
        NamedXContentRegistry xContentRegistry
    ) {
        this.settings = nodeSettings;
        this.client = client;
        this.threadPool = threadPool;
        this.xContentRegistry = xContentRegistry;
        this.clusterService = clusterService;
    }
    /**
     * Initialize the template registry, adding it as a listener so templates will be installed as necessary
     */
    public void initialize() {
        clusterService.addListener(this);
    }
    /**
     * Retrieves return a list of {@link IndexTemplateConfig} that represents
     * the index templates that should be installed and managed.