Add `materialKey` to `CesiumRasterOverlay`

[j9liu]

Depends on #385 so merge that first.

Fixes #401.

This PR adds the materialKey property to CesiumRasterOverlay and intends to be analogous to material layer keys in Cesium for Unreal. In the tileset materials, overlay parameters now have the following syntax:

• _overlayTexture_KEY
• _overlayTextureCoordinateIndex_KEY
• _overlayTranslationAndScale_KEY

Where KEY is the string in materialKey. On the native side, raster overlays store materialKey in their name so that they can be attached / detached from the correct variables.

The UI for CesiumRasterOverlay has been updated to reflect this property. However, instead of the typical text field, it displays a popup that reflects which keys are actually present in the material.

Finally, I also took the time to add this little warning in the CesiumRasterOverlay. It appears if the raster overlay is not attached to a Cesium3DTileset. (might be nice to use this in other components too!)

[kring]

This looks great! One small problem I noticed: it looks like this PR is accidentally undoing a cesium-native upgrade.

[j9liu]

@kring Fixed, thanks for the catch!

[kring]

Thanks @j9liu!