TypedSchema and Dependency should be generic

There's lots of places where it's challenging to add static typing where we have to deal with `Dependency` and `TypedSchema`, and we need to add assertions on their contained schemas. This is due to the fact that we maintain no connection between their `.schema_type` and types of `.schema`.

Having the ability to specify `Dependency[ProtobufSchema]` when working with protobuf-specific code would help greatly in this respect.