Fix driver key to known string

[zuckschwerdt]

Sets getDriverKey() to "PlutoSDR", sets getHardwareKey() to "ADALM-PLUTO", and returns "library_version" and "backend_version" with getHardwareInfo()

[zuckschwerdt]

Changes the Identification API's getDriverKey() and getHardwareKey() to return known fixed values.

This is in line with how other drivers report the keys and enables reliable driver/device identification in applications.

S.a.

https://github.com/pothosware/SoapyPlutoSDR/blob/master/PlutoSDR_Settings.cpp#L64 https://github.com/pothosware/SoapyRTLSDR/blob/master/Settings.cpp#L134 https://github.com/pothosware/SoapyAirspy/blob/master/Settings.cpp#L111 https://github.com/pothosware/SoapySDRPlay/blob/master/Settings.cpp#L131 https://github.com/pothosware/SoapyHackRF/blob/master/HackRF_Settings.cpp#L97

[guruofquality]

Great idea