Make model validation info/errors easily visible without having to look at logs

Currently we emit debug and warning messages when building and validating the model. However, these are missed if the developer is not looking at logs, or is filtering messages. With document modeling, complex types as well as owned types, relationships defined by path, etc. there are more and more places where we want to give feedback on choices made in the model.

Possible ways to make this more visible:

- Analyzer (Not sure it is feasible, but this would be awesome.)
- `dotnet ef validate` or similar
- A runtime method ("ValidateModel") that would be used in the implementation `dotnet ef validate` but can be called directly.