Add DigitalInputProtocol, GpioDigitalInputDriver

[flxzt]

Currently it is only possible to specify GPIO's as outputs see here.

I am proposing to add a DigitalInputProtocol together with a GpioDigitalInputDriver that configures GPIO pins as input only through sysfs (or switch to the newer devfs/libgpiod based GPIO interface while at it, because the sysfs one is deprecated). This could ensure that there won't be any shorts when the pin is connected to an output of the DUT.