Windows Path Support

[maspe36]

This PR should fix the windows build failure for rclrs in ros2-rust/ros2_rust#556

--- stderr: rclrs
error: unknown character escape: `p`
 --> C:\workspace\build\rclrs\debug\build\ros-env-9ae9ad711b590cc7\out/interfaces.rs:5:13
  |
5 |  include!("\pixi_ws\ros2-windows\share\action_msgs\rust\src\msg.rs"); pub mod rmw { use crate::builtin_interfaces;
  |             ^ unknown character escape
  |
  = help: for more information, visit <https://doc.rust-lang.org/reference/tokens.html#literals>
help: if you meant to write a literal backslash (perhaps escaping in a regular expression), consider a raw string literal
  |
5 |  include!(r"\pixi_ws\ros2-windows\share\action_msgs\rust\src\msg.rs"); pub mod rmw { use crate::builtin_interfaces;
  |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We were not handling paths for windows correctly. Now we get the absolute path to the generated .rs files and include those. This should handle all platform differences (similar to canonicalize()), except std::path::absolute does not resolve symlinks.
https://doc.rust-lang.org/stable/std/path/fn.absolute.html

---

I am testing this PR with this draft PR in rclrs. Ideally this repo should have CI as well, but this requires more effort and shouldn't be rushed solely for this change.
ros2-rust/ros2_rust#623

[esteve]

@maspe36 thanks! I'll rebase ros2-rust/ros2_rust#556 once I merge this and then review it.

[maspe36]

Verified windows builds work with this change!
https://github.com/ros2-rust/ros2_rust/actions/runs/24309522618/job/70976627263?pr=623