Notes for Reviewer

Fix bug where string literals were not null terminated and clean up implementation:

1. The trait and macro were renamed (trying to be more precise):
   1. Trait: AsCStr which provides as_const_cstr() method, const alluding to the fact it is in .rodata
   2. Macro: CStrRepr so #[derive(CStrRepr)] reads more naturally
2. The trait explicitly returns the CStr type
3. The test now properly asserts the generated strings are valid c strings:

   assert_eq!(v1.as_const_cstr(), c"custom variant one");
   assert_eq!(v2.as_const_cstr(), c"variant two");
   

4. Move CStrRepr macro into iceoryx2-ffi-macros since it is only used by iceoryx2-ffi

Unrelated: Also fix typo in existing assert_that message.

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

Closes #491