Feature/observable-sorting-paging

[einari]

Added

• Support for paging and sorting as a frontend concern for observable queries. This includes all the way from the proxy generators and down into the backend implementation for MongoDB.
• Added visibility of query parameters related to sorting and paging, seeing that they're automatically handled as a separate concern.
• Adding error codes to Swagger definitions for Commands and Queries.

Changed

• MongoCollectionExtensions convenience methods for observing mongo collections are now returning ISubject<> instead of Task<ISubject<>>. The reason for this is that ISubject<> as a construct can be created synchronously and we now instead run the actual query and the Mongo Watch in a separate task that calls OnNext() on the subject.
• Swagger schema filter added that will now show enums as string names, rather than integer values. The JSON serializer should support deserializing enums as strings.

Fixed

• Migrated from Moq to NSubstitute