Add interceptors for .NET HTTP

[xiazcy]

Currently targeting the feature branch for easy diff checking, will rebase and update once the non-streaming HTTP PR get merged.

[kenhuuu]

There's a bit of a design issue with the way this is currently implemented. The HttpRequestContext allows for setting/getting the body, but the body is immediately serialized before the interceptors run and the only supported serializer is GraphBinary. This means that users really have no chance to modify the body at all and they can only read it, otherwise, their interceptor would need to deserialize the body, make changes and then re-serialize.

In my opinion, one of two things need to be changed:

• split Auth away from interceptors and remove Body from interceptors to make it clear that only headers can change in interceptors but Auth can read the body
• Don't serialize immediately and put the body in a form that is easily modifiable, but this will require more understanding from the user about how the system works. This is effectively what the Java driver does.