samples: subsys: usb: midi: add new sample

Add a sample application that demonstrates how to use the new USB-MIDI 2.0
device class. This shows how to set up the device tree, how to exchange
MIDI data with the host, and how to use the data accessors provided by
the new API.

Signed-off-by: Titouan Christophe <moiandme@gmail.com>