Integrate camera flight with timeline

This question [came up on the forum](https://groups.google.com/d/msg/cesium-dev/ww9iLRBYs-g/UPmWFERjDAAJ) and I was surprised that there was no way to hook a camera flight into the timeline. I posted what I think would be a possible workaround there. But since doing this properly might take a bit of restructuring of the camera code, I thought it might be worth having the library itself handle this.

It is a bit strange that the camera uses TweenJS to provide access to common easing functions, but this is not available or exposed to the entity system (https://github.com/AnalyticalGraphicsInc/cesium/issues/7597). Entities instead rely on interpolating a path defined by control points, as opposed to easing between individual points. Is the reason for this just that the camera was written before entities existed?

I think this is a good opportunity to suggest making this API more consistent. Since we already ship with TweenJS it might be nice to expose it to entities too. Or we could just remove the TweenJS dependency and have the camera use the same dynamic property system as entities.

Thoughts?

Begin with the linked forum discussion and issue #7597, then trace the camera flight, timeline, entity interpolation, and TweenJS integration described here. Clarify whether the intended outcome is exposing TweenJS, replacing it with the dynamic property system, or another API; done requires a decided design and corresponding behavior.

Written by the indexing model from the issue text.