Create special types that parse singletons as list for user convenience

[apirogov]

Technically, we probably could provide for List and Set something roughly equivalent to:

OneOrList[T] = Union[T, List[T]] from input point of view, but = List[T] from instance point of view.

That means, it should normalize the input to be a (possibly singleton) list (or set, respectively).

This still would need to play nicely with subtypes, pydantic doing the right thing, type checker not complaining etc, so needs a bit of thinking how to be done right.

It should not be the default, but more of a compatibility hack to extend a "single item" field with a "collection" variant in a backward-compatible way (ideally compatible with issubtype, or at least "semantically" and "practically")