Use System.Text.Json for JSON serialization/deserialization

[wazzamatazz]

At the moment, the Data Core HTTP client (and derived types) use Microsoft.AspNet.WebApi.Client and Newtonsoft.Json to send and receive JSON payloads in calls to the Industrial App Store and Data Core APIs.

Removing the dependency on the legacy Web API client libraries and Newtonsoft, and adding in JSON serializer support from System.Text.Json would give performance and future-proofing benefits.