Use Discriminated Union Types for TaskResponse interfaces

[stevengum]

Per Teams Hackathon feedback

(And for other applicable areas)

[enhancement]

[stevengum]

Closing as we've loosened the return values for the TaskFetch and TaskSubmit methods in the TeamsActivityHandler to take the outermost envelope. (TaskModuleResponse)

We're instead going to use a Factory in the Samples for 4.6 with the intent of moving these factories into the libraries in 4.7.

FYI @clearab