Switch from rcpputils::fs to std::filesystem

[christophebedard]

Part of https://github.com/ros2/rcpputils/issues/164

I split the PR into two commits: internal changes and API changes.

[christophebedard]

I opened this as a draft because I'm not sure how to deal with the public API change for rclcpp::get_logging_directory(). I changed the return type from rcpputils::fs::path to std::filesystem::path because we can't have both, but they're functionally the same. I can't find any non-test calls to rclcpp::get_logging_directory() in the ROS 2 core.

The "cleanest" option is probably to deprecate the current function and create a new one with a different name, but that's unfortunate.

[christophebedard]

Superseded by #2579