Rename Repository#modelPrototype, add Repository#modelSchema

I don't know why `Repository#modelPrototype` is called the way it is, but it's clearly wrong as it's actually referring to the model itself (i.e. the constructor), not the model's prototype.

I took the liberty of renaming it to `Repository#model` and adding an accessor property that logs a deprecation warning. This allows maintaining backwards compatibility while discouraging the use of `Repository#modelPrototype` (so we can use it for something else in the future).

I also added a getter for the model's schema, as that is something one may want to use occasionally (e.g. for `queryParam` or `pathParam` validation) and currently you have to access it via the model's (actual) prototype, which can be unwieldy.

We should make sure nothing in Foxx (or the bundled apps) uses `modelPrototype` before we merge this.