Support in Unreal for Cesium ion servers running in single-user authentication mode.

[azrogers]

Requires CesiumGS/cesium-native#841.

As described in the above ticket, we don't currently support Cesium ion Self-Hosted servers that are running in single-user authentication mode. This is inconvenient, especially since single-user mode is both the easiest to setup and the default authentication mode in Cesium ion Self-Hosted.

This PR changes the behavior of Cesium for Unreal to request the current authentication mode from the /appData endpoint before connecting, ignoring the authentication flow and token options if in single-user mode.

[j9liu]

Actually @azrogers, I'll look into the KTX2 related errors since they're out of scope for this PR, and they may have been related to the ktx -> ktx_read changes.

[j9liu]

@azrogers Thanks for resolving the suggestions! I think you're still missing a catch for adding assets from the single user server. (It works in Unity, but I don't see equivalent code here)

[azrogers]

@j9liu added check!