usb: device_next: support vendor request with recipient device

Allow the user to register a vendor request node identified by the
vendor code (bRequest) and containing two callbacks to handle the vendor
request. The device stack uses the vendor request node to call the
vendor request callbacks when it receives a request of type Vendor,
recipient Device, and bRequest value equal to the vendor code.

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