Add request types to allow for mediator pattern invocation of tasks

[jviau]

This PR introduces the mediator pattern for invoking of orchestrations and activities. This allows for users to invoke activities and orchestrations via strongly-typed request objects and also to build up complex invocation patterns through additional TaskOrchestrationContext extension methods and more derived request types. Some examples would be:

1. A polling pattern. We can introduce a ILongRuningActivityRequest<TResult> and a RunAsync extension method for it which will handle enqueuing a polling orchestration and polling activity.
2. A paging activity pattern - we can leverage our AsyncPageable<T> classes to allow for paging over activities via IAsyncEnumerable<T> in your code.

This will also replace the code-generation approach for strongly typed orchestration/activity invocations. In my opinion, I prefer this route over code-generation. It is more flexible and avoids function-name conflicts. We can still explore how we can leverage code-generation to add value to this pattern though - but it will always be an optional addition.

resolves #73

TODO:

• unit tests
• samples
• DurableTaskClient extension method for enqueuing orchestrations.