Rounded Cube sample for KHR_xmp_json_ld extension.

[weegeekps]

This adds the Rounded Cube sample to the samples repository for the upcoming KHR_xmp_json_ld extension.

[cx20]

To be consistent with other samples, it is better to name the sample model file the same as the folder name is a good idea.

Folder Name	File Name
Duck/glTF	Duck.gltf
Duck/glTF-Binary	Duck.glb

Also, to keep things simple, I think it would be better to put the .bin file that this model refers to in the same folder.
This is because some viewers This is because some viewers cannot display correctly when using a relative path.
For example, the Khronos glTF Sample Viewer fails to display the .bin file if it is in a different folder.