Remove only, exclude, many, and unknown parameters of Nested and require callables to return Schema instances

[sloria]

Proposing the following 2 changes to fields.Nested:

(1) drop support for passing callables that return schema classes and dicts (e.g. lambda: CSchema) and only support callables that return instances (lambda: CSchema()).

Before:

        nested: (
            Schema
            | SchemaMeta
            | str
            | dict[str, Field]
            | typing.Callable[[], Schema | SchemaMeta | dict[str, Field]]
        ),

After:

        nested: (
            Schema | SchemaMeta | str | dict[str, Field] | typing.Callable[[], Schema]
        ),

And (2) deprecate the only, exclude, many, and unknown parameters of Nested.

Final end state of fields.Nested would be

    def __init__(
        self,
        nested: (
            Schema | SchemaMeta | str | dict[str, Field] | typing.Callable[[], Schema]
        ),
        **kwargs: Unpack[_BaseFieldKwargs],
    ):

Why?

It would vastly both the usage and the internal implementation of Nested. This eliminates a class of bugs where the nested argument conflicts with only, exclude, many, and unknown.

See #2165 for an example of this.

[lafrech]

I agree it is strange to accept an instance and modifiers applying to that instance. I don't like modifying instances. This is something we may want to prevent (#1281).

But it may be useful for nested schemas passed by name as string, or by class. (I don't see the point of accepting a class, we could require an instance, but I see the point of accepting a schema name as string, which ultimately falls in the same case as a class).

Regarding schemas passed as callable, it may be surprising that only instances are accepted, but if it makes our job easier, I don't mind dropping the class case and require an instance. Doing the instantiation in the callable is not too much to ask. (We never accepted a callable returning a schema name as string, anyway.)