Add example of using TinyUSB CDC with extra stdio_usb iface

[kilograham]

see: https://forums.raspberrypi.com/viewtopic.php?t=363831

it'd be nice to have an example where we use one of the users own CDC for stdio. Note the SDK sort of handles this auto-magically if you've explicitly linked tinyusb_device, but maybe we can also show choosing which CDC to use (which may not yet be possible - perhaps not necessary, or we could add a define for the index)