Implement transfer mode switching (bulk/isochronous)

Allow switching between bulk and isochronous USB transfer modes:
- `facecam-ctl control set transfer_mode bulk`
- `facecam-ctl control set transfer_mode isochronous`

Isochronous mode has been reported to resolve freezing on some systems. Currently only settable via Camera Hub.

Depends on HID protocol decoding (#12).