Refactor zenoh_config.hpp so we can reuse it in the router

[clalancette]

We have code in https://github.com/ros2/rmw_zenoh/blob/rolling/rmw_zenoh_cpp/src/detail/zenoh_config.hpp and https://github.com/ros2/rmw_zenoh/blob/rolling/rmw_zenoh_cpp/src/detail/zenoh_config.cpp that is currently used to allow the user to configure the session configuration when using rmw_zenoh_cpp.

We should also use that code to be able to configure router configuration as well. That way, users will be able to launch the router locally with either the default config, or one they provide. The code probably needs some refactoring to be able to use it for that purpose.

This is a follow-up task from #101 .

[Yadunund]

Closed via #119