Add KHR_gaussian_splatting and SPZ_2 extension support

[gugu23456789]

This PR adds support for the Khronos glTF extensions:

• KHR_gaussian_splatting
• KHR_gaussian_splatting_compression_spz_2

Changes

• Added GaussianSplatExtension and GaussianSplatSpzCompression structures in types.hpp
• Added JSON export support in fastgltf.cpp
• Added FASTGLTF_ENABLE_KHR_GAUSSIAN_SPLATTING compile flag (default ON)

Usage

// The extension is enabled by default
// To disable: -DFASTGLTF_ENABLE_KHR_GAUSSIAN_SPLATTING=OFF

// Access the extension
if (primitive.gaussianSplat) {
    if (primitive.gaussianSplat->spzCompression) {
        auto bufferView = primitive.gaussianSplat->spzCompression->bufferView;
    }
}

Specification

https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_gaussian_splatting
KhronosGroup/glTF#2531 (SPZ_2 extension, pending merge)

Testing

• Tested with SPZ to GLB conversion
• Verified JSON output format matches specification

[spnda]

I think there are missing commits, since only the exporting code and the type definitions are included. Otherwise, I'd be happy to merge this. Also, the extension has been merged so there's no need to hide it behind the compile-time flag.