Calling/Considering it "sunglasses" doesn't make it out of the place in the Medieval era. After all, glasses are not new in the era as there are windows and people did wear spectacles (monocle). So, just take any skilled craftsman, take glass as raw material, tint it and voila, you have a "sunglass". However, that is not a "sunglass" but a magical item called Sunglasses of the Rescuer. But to be fair, the name is a little bit anachronistic.
More details in this Reddit post: https://www.reddit.com/r/mushokutensei/comments/14r1dx1/where_did_fitz_get_those_modern_looking/