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/test/java/org/elasticsearch/xpack/core/template/IndexTemplateRegistryTests.java
private TestRegistryWithCustomPlugin registry;
    private ClusterService clusterService;
    private ThreadPool threadPool;
    private VerifyingClient client;
    @Before
    public void createRegistryAndClient() {
        threadPool = new TestThreadPool(this.getClass().getName());