Allow navigations to be specified when using the basic model builder APIs #781
Description
The relationship builders on the convention model builder directly use navigations and hence allow/require them to be specified. However, the basic model builder does not have these relationship builders since they depend on by-convention creation of FKs to work. This means that in the basic model builder the only way to specify relationships is through the entity level ForeignKey API, but this currently does not contain any way to specify navigations.
It is not completely clear how navigations should be added to this API. From a model perspective each relationship can have zero, one, or two navigations, and one of these lives on a different entity from the FK. If the basic fluent API were to follow the model closely, which was the general idea, then each entity should have a Navigation method, but this would need to somehow reference an FK possibly on a different entity type. It might be better instead to have the navigation methods hang off the ForeignKey API and allow one or both navigtions to be specified explicitly that way, even though this is a bit of a departure from how the model is structured.