Include generics in discriminated union schemas

[tpoliaw]

• Restrict subclasses of typed unions
• Allow arbitrary types to be included in 'StrictConfig' types

Fixes #156

Instructions to reviewer on how to test:

1. Create set of subclasses of a decorated base class:

@discriminated_union_of_subclasses
class Base(Generic[T]):
    pass

@dataclass
class Child(Base[T]):
    foo: T

2. Try and deserialise a serialised Child as a Base:

one = TypeAdapter(Base[int]).validate_python({"type": "Child", "foo": "42"})
assert one.foo == 42 # <== 42 instead of "42" as type should be converted

Checks for reviewer

• Would the PR title make sense to a user on a set of release notes

[coretl]

From a skim this looks fine to me