[Enhancement] Make APISettings a generic class.

[PedroPerpetua]

Some modules designed to work with DRF subclass the restframework.settings.APISettings class to implement their own custom settings (for example, djangorestframework-simplejwt). This class has the expected settings keys "hardcoded" into the class itself, making subclasses have to type-ignore when using their own Settings TypedDict or have some incompatible override.

The class could be made generic so subclasses pass their own TypedDict of settings.