Add function to get non-ROS arguments

[nnmm]

When the context receives command line arguments, it only parses the ones between --ros-args and a --. The context could have a function to return a list of command line arguments not parsed by ROS, for further processing with clap or so.

This should be done using functions from https://github.com/ros2/rcl/blob/foxy/rcl/include/rcl/arguments.h, and the rcl_arguments_t instance stored in the rcl_context_t.

[Nizerlak]

I think this issue can be closed because of #210 merge.

[nnmm]

True!