Add a way to override a type within a model for all fields

[apirogov]

When subclassing a schema, e.g. to say "use MyPerson instead of Person in all fields"

Otherwise each field must be overridden explicitly, which is error prone or confusing.

Again the question arises: Decorator? Meta-class? this should not be too annoying to use.

Maybe the decorator should look like

@specialize_schema(schemaorg.Person, rocrate.Person, fields={"names", "where", "to", "substitute"})
class MyThing(schemaorgbasedThing):
 ...

and should

• raise an exception if new type is not a subclass
• otherwise substitute this schema in all (including inherited) type hints

In all other cases (not specializing to a subtype) it should remain a manual type hint, otherwise it would be to easy to wreak havoc.

[apirogov]

Given the current state of things, it should be possible, using an approach similar to the make_mandatory decorator and combining it with a type hint traversal to substitute the type.

Main question is to make sure that this also works fine with Partial and Fields and the cached type hints.