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
logger.error(
                                "error adding ingest pipeline [{}] for [{}], request was not acknowledged",
                                pipelineConfig.getId(),
                                getOrigin()
                            );
                        } else {
                            logger.info("adding ingest pipeline {}", pipelineConfig.getId());
                        }
                    }
                    @Override
                    public void onFailure(Exception e) {
                        creationCheck.set(false);
                        onPutPipelineFailure(pipelineConfig.getId(), e);
                    }
                },
                (req, listener) -> client.execute(PutPipelineAction.INSTANCE, req, listener)
            );
        });
    }
    /**
     * Called when creation of an ingest pipeline fails.
     *
     * @param pipelineId the pipeline that failed to be created.
     * @param e The exception that caused the failure.
     */
    protected void onPutPipelineFailure(String pipelineId, Exception e) {
        logger.error(() -> format("error adding ingest pipeline template [%s] for [%s]", pipelineId, getOrigin()), e);
    }
}