Create a new product tag (version=1)

• product: which product
• k: which key for the tag
• v: which value to set for the tag
• version: none or empty or 1
• owner: none or empty for public tags, or your own user_id

Be aware it's not possible to create the same tag twice. Though, you can update a tag and add multiple values the way you want (don't forget to document how); comma separated list is a good option.