Fix #1410 by adding some clever inheritance tricks to make OData clients work properly

I love it when my first hunch works out perfectly :).

By changing our usage of the FeedContext&lt;T&gt; to be a base-class rather than using it directly, we bypass the problem with "FeedContext`1" (which is a Generic Type Name). Now, the entity context type is either V1FeedContext or V2FeedContext (but those types are still just simple shims around FeedContext&lt;V1/V2FeedPackage&gt;)

Fixes #1410