- Allows classes to handle vendor requests.
- Allows classes to use a single handler for multiple interfaces.
- Allows classes to access the other events (previously only `reset` was available).
This brings it inline with the other embassy-usb descriptor APIs and allows it to integrate well with the Builder to allow class constructors to add MS OS descriptors.
Also adds a `usb_serial_winusb` example to demonstrate how to use the API.
