Send extension fields as part of datasetmessage and enable free form configuration of extension fields (move to Variant value)

[marcschier]

          I moved the filling of extra extension fields into the encoder.  Need to still support extension fields in PubSub messages and configuration from control plane, will put on backlog.

Originally posted by @marcschier in #1928 (comment)

[marcschier]

Customers want the ability to add machine or engineering unit context to messages. Extension Fields in OPC UA Pub Sub support this. The plan is to add full support to OPC Publisher. Customers will be able to add extension fields to pn.json per endpoint/per datasettag. These will become fields of the dataset as if they originate from the OPC UA server and thus allow a consumer to understand the context of the message.