Feature: Add SchemaOpts to opt-in to validation

[RookieRick]

Currently (technically, when 2.0.1 is merged and released) it is possible to opt in to response validation at the schema level by inclusion of `RequireOnDumpMixin.
While this works, a more natural fit would be to allow this to be directly specified in the schema definition itself by including a "custom class Meta option" as described here: https://marshmallow.readthedocs.io/en/stable/extending.html