Add support for custom ellipsoids

[azrogers]

Currently, Cesium for Unreal is hardcoded to use the WGS84 ellipsoid. This change adds support for specifying other ellipsoids by creating assets for them and assigning them to georeferences.

Closes #366

[azrogers]

@csciguy8 Made GetNativeEllipsoid return a const ref, made UCesiumEllipsoid store a TOptional instead of a TUniquePtr, fixed the OnEllipsoidChanged event (apparently the handler needs to be marked with "CallInEditor"), and added a method that reloads any georeferences using an ellipsoid asset when that ellipsoid asset is saved. Should be good to go!

[csciguy8]

Looks great @azrogers!

Will merge once CI passes (almost done).

Kudos for catching the event when the ellipsoid data changes. UCesiumEllipsoid::PostSaveRoot does not look like an easy or obvious way to handle this