Use OPC UA JSON Encoding for PubSub Payload

[GoetzGoerisch]

The current JSON Payload is OMP own mapping/encoding.

OPC UA defines with Part 14 also the PubSub communication over transport protocols like MQTT.

For JSON the OPC spec defines an interoperable mapping to be consumed by cloud and data analytics applications.
https://reference.opcfoundation.org/Core/docs/Part14/6.3.2/

As a starting point please have a look an the reference implementation by @barnstee.
https://github.com/barnstee/UA-CloudPublisher

@veithammerstingl for OPC UA for Machine Tools a prototype implementation is already available at: https://github.com/umati/Sample-Server/tree/pubsub