Device message serialization improvements

[pral2a]

Currently, the MQTT API accepts readings in two formats:

• device/sck/%s/readings used by a device to publish one or multiple sensor readings in JSON format
• device/sck/%s/readings/raw used by a device to publish sensor readings in `raw`` non-standard format (see custom parser)

Finding an efficient serialisation yet standard method to replace raw might be relevant for platform growth and standardisation.

That will allow forwarding MQTT messages to third-party platforms in a well-defined standard instead of a custom "broken" JSON.

Potential standards:

https://msgpack.org/index.html
https://protobuf.dev/