Represent base class for context - Rights or Processing context. Both of them is a list of Evidence.
Adding additional properties to EngineContext in the case of ProcessingContext. AllowCache property - indicate if for such ProcessingContext allowed caching of transformed values. (For example, in case of probabilistic transformation we shouldn't cache values).
Abstract base class for ContextProvider. Implement base functionality for retrieve rights and processing contexts from the dictionary by key. Contains two abstract methods: GetRightsContexts and GetProcessingContexts to implement into specific provider (File, DB, etc).