Cherry pick CesiumIonClient changes into v0.27.x

[kring]

This cherry picks the commits from the following PRs into the v0.27.x branch:

• https://github.com/CesiumGS/cesium-native/pull/744
• https://github.com/CesiumGS/cesium-native/pull/772
• https://github.com/CesiumGS/cesium-native/pull/773

Plus some odds and ends to adapt for other reorg changes that happened in v0.28.x.