WMTS Support addition

Contribution for WMTS 1.0.0 support in cesium JS.

The spec.js file (Specs/Scene/WMTSImageryProviderSpec.js ) still needs to be substantially filled.
Some tutorials or examples of use should also be added.
We're not linking to `FrameState` because it's private now.

@pjcozzi does that mean we should change these callbacks to not take a `FrameState` anymore?  The intention was to allow the alpha, etc. to vary with time, camera location, etc. but I'm not aware of anyone using the feature.