[Backport iron] populate imu covariances when converting (#375)

[wittenator]

🎉 New feature

Closes #539

Summary

I cherry-picked the commit that fills the covariances in the IMU messages on top of the iron branch.

Test it

None

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] Added example and/or tutorial
• [ ] 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.

[wittenator]

@ahcorde Could you rerun the tests? I think them failing is unrelated to the changes I did.

[ahcorde]

@wittenator CI is broken, this PR https://github.com/gazebosim/ros_gz/pull/541 should fix it.

[ahcorde]

do you mind to merge with iron ? CI should be fine