serial_hid: implement serial over HID transport

serial_hid: implement serial over HID transport

Do implement the transport methods for serial communication underneath
the common layer, by communicating HID requests and payload data by
means of HIDAPI library calls.

This commit adds the common logic of serial-over-HID communication and
implements the full internal serial transport API, including reception
in the background. But it does not yet support a single HID chip (which
each run their own proprietary protocol).

The implementation works with either hidapi-libusb or hidapi-hidraw
variant of the HIDAPI library, but was only tested on Linux.