[#264] Expose file descriptor in C++ bindings

[elBoberido]

Notes for Reviewer

This PR adds an API to expose the file descriptor of the listener in the C++ bindings. This is required if the listener needs to be used with a third party event loop.

The access does not happen directly but by a wrapper type to signal the unsafety of the operation.

Pre-Review Checklist for the PR Author

1. Add sensible notes for the reviewer
2. PR title is short, expressive and meaningful
3. Relevant issues are linked in the References section
4. Every source code file has a copyright header with SPDX-License-Identifier: Apache-2.0 OR MIT
5. Branch follows the naming format (iox2-123-introduce-posix-ipc-example)
6. Commits messages are according to this guideline
   • Commit messages have the issue ID ([#123] Add posix ipc example)
   • Commit author matches Eclipse Contributor Agreement (and ECA is signed)
7. Tests follow the best practice for testing
8. Changelog updated in the unreleased section including API breaking changes
9. Assign PR to reviewer
10. All checks have passed (except task-list-completed)

Checklist for the PR Reviewer

• Commits are properly organized and messages are according to the guideline
• Unit tests have been written for new behavior
• Public API is documented
• PR title describes the changes

Post-review Checklist for the PR Author

1. All open points are addressed and tracked via issues

References

Relates to #264

[elBoberido]

@elfenpiff @orecham I'm open to suggestions. I tried to be close to Rust and the call to access the file descriptor would look like auto fd = listener.unsafe().file_descriptor().expect("It works");

[elBoberido]

Already accomplished with #524