Support KHR_technique_webgl extension for glTF

[pjcozzi]

@ggetz @lilleyse for after Draco, #5120

Please track and participate as needed in the spec discussion here: KhronosGroup/glTF#1217

Update both Cesium and gltf-pipeline.

[pjcozzi]

@ggetz OK to close this?

[OmarShehata]

@ggetz it looks like the extension was renamed from KHR_technique_webgl to KHR_techniques_webgl at some point, (it doesn't have the s in 1.49) but I don't see a breaking change announcement for it in changes.md.

This is breaking some people's tilesets (see forum thread). Should we just check for both or should we have had a deprecation period? I'm not sure what all went into this rename so apologies if I'm missing some context here.

[ggetz]

The extension name itself changed after he initial draft. They should update their tilesets since it's no longer valid. I think checking ad throwing a oneTimeWarning which we eventually remove would work. @lilleyse sound good?

[OmarShehata]

I just spoke with @lilleyse offline, and it sounds like even though it was in 1.49, it was never officially supported, and that it's not just a naming change, that the old tilesets wouldn't really work out of the box, so it shouldn't be something we can support.

Perhaps the best course of action here is to update those old tilesets with obj2gltf ? (Assuming the source obj still exists).