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
executeAsyncWithOrigin(
                client.threadPool().getThreadContext(),
                getOrigin(),
                request,
                new ActionListener<AcknowledgedResponse>() {
                    @Override
                    public void onResponse(AcknowledgedResponse response) {
                        creationCheck.set(false);
                        if (response.isAcknowledged() == false) {
                            logger.error(
                                "error adding legacy template [{}] for [{}], request was not acknowledged",
                                templateName,
                                getOrigin()
                            );
                        }
                    }
                    @Override
                    public void onFailure(Exception e) {
                        creationCheck.set(false);
                        onPutTemplateFailure(templateName, e);
                    }
                },
                client.admin().indices()::putTemplate
            );
        });
    }
    private void putComponentTemplate(final String templateName, final ComponentTemplate template, final AtomicBoolean creationCheck) {
        final Executor executor = threadPool.generic();
        executor.execute(() -> {
            PutComponentTemplateAction.Request request = new PutComponentTemplateAction.Request(templateName).componentTemplate(template);