usb: device_next: implement USB DFU class for the new device support

This new implementation is written from scratch and is not tied to the
image manager and MCUboot. It allows the user to define their own
backend and use a simple macro to instantiate an image. On the USB side
this is represented by an interface. The number of possible images is
configurable using the Kconfig option, and is a fairly inexpensive
approach since it only changes the size of the pointer array. The number
of images is only limited by the number of possible interfaces in a
configuration. The class implementation does not support multiple
instances, as there is no real use for it. However, it does provide two
class instances, one for runtime mode and one for DFU mode. The switch
from runtime to DFU mode can only be performed by the user application,
i.e. the application receives a notification when the host wants to
switch to DFU mode, and then the application can disable the runtime
configuration and enable the DFU configuration. This implementation does
not support switching to the DFU mode by bus reset issued by the
host.

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