Allow programmatic configuration of unicast relays.

[mbeards]

This change allows users to configure relays from code without having to setenv(GZ_RELAY).

🎉 New feature

Summary

This change allows users to configure relays from code without having to setenv(GZ_RELAY).

Supersedes https://github.com/gazebosim/gz-transport/pull/497

Test it

This can be tested by starting a Node across a boundary where multicast isn't enabled, and setting the relay IP via NodeOptions.

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.