C++ binding: Allow to re-use ConfigKey::parse_string() for Option class

C++ binding: Allow to re-use ConfigKey::parse_string() for Option class

Split the data type detection from the actual data type conversion in
the ConfigKey::parse_string() method. Allow the Option class to re-use
the public ConfigKey method, to share the maximum amount of data type
conversion code for both Driver and InputFormat option specs.

This is the least intrusive yet most reliable and consistent approach in
the libsigrok C++ binding that allows applications to support driver scan
and input file format options from command line arguments.