Prototype/ordering

[lu-pl]

Concerns #171.

This draft implements a prototype for field-based result ordering.

QueryParameters now implements a metaclass-ish hook for dynamically assigning an Enum of order-able model fields at model class parametrization time. This leads to improved OpenAPI docs, see my initial metaclass draft. Note that this is done with __class_getitem__ now, which basically wraps metaclass behavior.

Also see the updated examples.