More options for flyTo, like space around target

For aesthetic reasons, it's sometimes desirable to have some space around the target after calling `viewer.flyTo`. This can be achieved with the `offset` option, but it's hard to determine the appropriate offset for targets of different sizes.
The API document says: "If the range is zero, a range will be computed such that the whole bounding sphere is visible."
Maybe we can have a `rangeMultiplier` kind of option that applies to the calculated range?
Or just expose a function that returns the bounding sphere of all those types of targets and let us do whatever we want to it.

Also, I don't want to get too close when the target is small. It would be nice if `flyTo` respects `ScreenSpaceCameraController.minimumZoomDistance` as requested by #9620, but I prefer a separate option (allowing the user to further zoom in if they like).

One more thing, doc for `Camera.flyTo` says that if `duration` is omitted, "Cesium attempts to calculate an ideal duration based on the distance to be traveled by the flight." This is a cool feature, but to me the calculated duration is kind of long. I hope I can specify a multiplier to the calculated duration, rather than a constant custom duration.

Start with the documented viewer.flyTo and Camera.flyTo APIs, focusing on the existing offset, range, duration, and ScreenSpaceCameraController.minimumZoomDistance behavior referenced in the issue and #9620. Clarify which of the requested capabilities is in scope, then define completion around one agreed API behavior and its documented result.

Written by the indexing model from the issue text.