Switch CesiumGltf to use std::vector<std::byte> rather than std::vector<uint8_t>

[kring]

It probably doesn't make much practical difference, but std::byte is slightly clearer semantically.

As suggested by @Samulus in #106.

[javagl]

I assume that this will not only refer to CesiumGltf, but to ~"all" places where uint8_t was used (in a vector or span) to represent "bytes without inherent meaning".

[javagl]

This was addressed in #148 and CesiumGS/cesium-unreal#189 . There are a few more places that I expected where uint8_t was used to transport raw bytes. If any of them should remain uint8_t (despite being raw, non-arithmetic data), I'll update that accordingly.

[javagl]

Closed via #148