[Client .Net] Add more APIs for calling actions

[tusmester]

Add more client APIs for calling content actions. New apis should provide shortcuts (e.g. dedicated actionName parameter instead of a generic ODataRequest parameter).

DISCUSSION: specify the API list before starting to work on this!

• [ ] add missing CancellationToken parameter where it is possible
• [ ] ProcessWebResponse and related methods: add async responseProcessor method param
• [ ] add shortcut methods for POST requests in RESTCaller
• [ ] add object parameter for post data (instead of string)
• [ ] add methods to the Content class static/instance API
  • [ ] action requests (GET/POST/PATCH) (with actionName parameter)
  • [ ] Exists (with id/path and version (!) parameter) for existence check
  • [ ] simplified download (by content id/path, target stream)
  • [ ] ...