Allow the plugins to set additional properties to monitor points

[acaproni]

It would be useful if the plugin allows to set additional properties to the monitor points.
The converter mast pass such properties the the converted IASIOs that already support that feature.
TFs can already get the properties of the inputs

Properties must be i the same format used along the core i.e. a map of key, value strings.

Python plugin must support this feature too.

The Additional properties field in the JSON strings sent to the plugin topic must be optional to avoid backward incompatibilities.

This feature allows the plugin to send more information than the value of a monitor point.