Fix UserCommands services so that they return the actual status the executed command (backport #2999)

[mergify]

🦟 Bug fix

Summary

Previously, the services immediately returned true after adding
the command to the pending commands queue which meant that
the return statuses did not reflect the success or failure of
the subsequent execution of the commands.

• Builds on top of Refactor Transport service handlers UserCommands #2996
• Needs Fix deadlock in service requests when running local callbacks gz-transport#671

Needed for gazebosim/ros_gz#790

Checklist

• 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)
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This is an automatic backport of pull request #2999 done by [Mergify](https://mergify.com).