Check assets against Google rules before uploading

Checks assets against Google's published rules without writing anything: image URLs are downloaded and measured (format, file size, dimensions, aspect ratio for the slot), text is length-checked, and every URL is checked for being a valid https address. Use when: before google_add_image_assets or google_create_pmax_campaign, so the user learns about a wrong crop before a half-built campaign does. Do not use when: you want per-asset performance (google_get_asset_performance) or the account's existing assets (google_list_assets). This call is free, needs no connection to a campaign and changes nothing. It reports each asset as ok or with the exact reason it would be rejected. If you are unsure which account, campaign or period the user means, ask them — do not guess ids or dates.