Upload image assets from URLs

Downloads images from https URLs, checks them against Google's rules (PNG/JPEG/static GIF, at most 5120 KB, the aspect ratio and minimum size of the slot they are for) and uploads the ones that pass as image assets. Use when: the user gave you image URLs and you are about to build a Performance Max asset group or add image extensions. Do not use when: you have not seen the images pass google_validate_and_prepare_assets and the user has not confirmed which slot each one is for — an image in the wrong slot is rejected by Google with an unhelpful error. Nothing is uploaded unless every image passes: a half-uploaded set is harder to clean up than a refusal. Google silently drops a new name for an image whose bytes already exist, so re-uploading the same file is safe but keeps the original name. The upload creates assets in the account library; it does not link them to anything. Link them with google_add_sitelinks-style tools or by building an asset group. If you are unsure which account, campaign or period the user means, ask them — do not guess ids or dates.