Refactor remaining Content operations to use the new API #98

Currently there are several instance operations (Delete, CheckOut) on the Content class that use the old RESTCaller static api. Update these methods:

• create new overloads with cancellation support
• make them call the IRestCaller instance if available instead of directly calling the static RESTCaller api (this way we get testability and retry automatically). Keep static calls in case of old methods for compatibility reasons.