TypedSchema and Dependency should be generic

[aiven-anton]

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.