usb: device_next: Introduce speed specific configurations

USB High-Speed devices must be able to operate at both High-Speed and
Full-Speed. The USB specification allows the device to have different
configurations depending on connection speed. Modify the API to reflect
USB Specification requirements on what can (e.g. configurations) and
what cannot (e.g. VID, PID) be speed dependent.

While the class configurations for different speeds are completely
independent, the actual class instances are shared between operating
speeds (because only one speed can be active at a time). Classes are
free to provide different number of interfaces and/or endpoints for
different speeds. The endpoints are assigned for all operating speeds
during initialization.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>