Add project soversion information to backward library

[cottsay]

🦟 Bug fix

Summary

The library being installed doesn't have a SOVERSION like other libraries created in Gazebo projects, and is flagged as a packaging error by rpmlint.

This will result in a change to the library's filename on Linux, which will now become libgz-tools2-backward.so.2.0.0. This is handled correctly by the use of $<TARGET_FILE_NAME:backward> for referencing the library in the gz executable.

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[scpeters]

I've updated the debian metadata in https://github.com/gazebo-release/gz-tools2-release/pull/15 in response to this change (see also https://github.com/gazebosim/gz-transport/pull/479#discussion_r1482203535)