Provide a unified parser interface for pluginlib

[ARoefer]

I'd like to specify a simple c++ parser interface class like this:

class IGiskardParser {
      std::string get_language_name() = 0;
      std::string get_file_suffix() = 0;
      giskard_core::QPControllerSpec parseFromFile(const std::string& file) = 0;
      giskard_core::QPControllerSpec parseFromString(const std::string& string) = 0;
};

This interface would allow users (like me) to implement their own parsers and make them accessible through the pluginlib. I'd like to add a feature to my simulation, that automatically finds all installed parser plugins and is able to parse the respective files. This opens the giskard framework to experimenting with different and more complex languages. Please let me know what you think about this idea.