dap: add DAP backend USB

The backend uses a new USB device stack and bulk endpoints. Only a
single instance of the backend function is currently supported. Both DAP
packet processing and the new backend are prepared to support multiple
instances, but require a user interface and minor modifications.

New backen has similar functionality to what is provided by the function
in the sample samples/subsys/dap.

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