protocol/driver: add DigitalInputProtocol, GpioDigitalInputDriver

[flxzt]

Description

this adds a new protocol DigitalInputProtocol and a GPIO driver that implements it.

The DigitalOutputProtocol ABC now also inherits it, ensuring it can be used in all existing driver implementations.

Additionally the client now automatically uses DigitalInputProtocol when the command is io get.

By seperating the input from output protocol it is possible to make sure the GPIO line is configured in direction input in sysfs, which avoids shorts when reading out digital states from DUT's. Otherwise, one would have to rely on the circuitry of the GPIO line.

Tested on two Raspberry Pi's as exporter and DUT.

Note:

the sysfsgpioin.py part duplicates most of the sysfsgpio.py code but because of the way this code is executed by agentwrapper I thought it was cleaner to separate it into a new file.

Checklist

• Documentation for the feature
• Tests for the feature
• The arguments and description in doc/configuration.rst have been updated
• PR has been tested

closes #1457

[flxzt]

I think the way I implemented the behaviour of the client doesn't work well - if labgrid-client io get is used after labgrid-client io high/low, the sysfs direction is changed from out to in which can result in state low even if it was set high previously.

I'll revert to the previous behaviour where DigitialOutputProtocol is used for all client io subcommands.
So DigitalInputProtocol will only be available for usage with the library, which is also acceptable imo.