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
getOrigin()
                        );
                        creationCheck.set(false);
                    }
                } else {
                    logger.debug("adding ingest pipeline [{}] for [{}], because it doesn't exist", requiredPipeline.getId(), getOrigin());
                    putIngestPipeline(requiredPipeline, creationCheck);
                }
            }
        }
    }
    @Nullable
    private static PipelineConfiguration findInstalledPipeline(ClusterState state, String pipelineId) {
        Optional<IngestMetadata> maybeMeta = Optional.ofNullable(state.metadata().custom(IngestMetadata.TYPE));
        return maybeMeta.map(ingestMetadata -> ingestMetadata.getPipelines().get(pipelineId)).orElse(null);
    }
    private void putIngestPipeline(final IngestPipelineConfig pipelineConfig, final AtomicBoolean creationCheck) {
        final Executor executor = threadPool.generic();
        executor.execute(() -> {
            PutPipelineRequest request = new PutPipelineRequest(pipelineConfig.getId(), pipelineConfig.loadConfig(), XContentType.JSON);
            request.masterNodeTimeout(TimeValue.timeValueMinutes(1));
            executeAsyncWithOrigin(
                client.threadPool().getThreadContext(),
                getOrigin(),
                request,
                new ActionListener<AcknowledgedResponse>() {
                    @Override
                    public void onResponse(AcknowledgedResponse response) {
                        creationCheck.set(false);
                        if (response.isAcknowledged() == false) {