.Net client: Repository instance API

[tusmester]

Design a new API that works in a modern DI-oriented environment - instead of the current static Content API.

• a single Repository service instance that encapsulates:
  • the connection to the repository service (currently ServerContext)
  • authentication
  • content instance creation API, including custom business types (e.g. MyFolder). The goal is to create content items in a way that lets developers inject services easily.
  • load and query API
• CRUD operations should remain on the Content class
• Content class can be inherited (custom business objects, see above)
• all APIs should be async (with cancellation) and trace/log every operation
• current/legacy APIs should remain intact
• make the whole static RestCaller class obsolete when this is completed.