Add `source` entry for ai_msgs (humble)

[AndyZe]

This is a new package so I'm adding the source entry per https://docs.ros.org/en/humble/How-To-Guides/Releasing/Release-Team-Repository.html#create-a-new-release-repository

This service is intended for prompting AI models.

@brettpac @yassiezar

[marcoag]

I commented on https://github.com/ros/rosdistro/pull/42020 about the package naming, let's continue the conversation there and apply the outcome to the relevant distros afterwards.