Shared-Type Entities: Self-Identifying entity instance

[smitpatel]

Currently when user uses DbContext.Add API, we can look up entity type based on the clr type of the entity instance. That does not work for Shared-Type entities. Hence we currently disallow using context-level APIs for shared-types. User must use DbSet.
In order to solve this, we could allow shared type entity instances which are self-identifying by using one of the property on the instance to give us name of the entity type.

[smitpatel]

I still have commit somewhere.