External app camera / timeline sync

[genemerewether]

This may be similar to #2638, but a really handy functionality would be real-time, bidirectional camera and timeline sync with an external app.

I'm not sure the current state of controlling 3D space view camera / timeline from outside Rerun, but it would be good to have minimal latency there, like imperceptible to a user operating an external app while also looking at the Rerun UI.

In addition, it would be nice to be able to subscribe to / fetch / poll the state of the Rerun UI:

1. Altering the camera pose in a 3D space view would allow driving an external app's camera
2. Altering the position on the timeline would allow driving an external app's timeline

I forget if I ever got this working in Rviz, but something like: https://answers.ros.org/question/392097/controlling-rviz-camera-programmatically/ and https://wiki.ros.org/rviz_animated_view_controller

[Wumpf]

Also very related to:

• New API to request userspace-callbacks from interaction events in visualizer #2691