Add Metal Uploader

We'd like to add a function to upload textures to Metal along the lines of ktxTexture_vkupload.

Start by locating the ktxTexture_vkupload function and reviewing how its texture-upload behavior maps to the requested Metal uploader. Define the new function's scope and completion criteria around uploading textures to Metal, then verify the resulting API with focused tests or an example if the project provides one.

Written by the indexing model from the issue text.