usb: device_next: add BT HCI USB transport layer

Add Bluetooth HCI USB transport layer implementation for the new
experimental USB device support based on the existing implementation
subsys/usb/device/class/bluetooth.c.

This implementation, unlike existing one, contains the interface
descriptors for isochronous endpoints. Since we do not support voice
channels, these are just there to avoid issues with Linux kernel btusb
driver when this implementation is part of a composite configuration.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>