Allow granting products using the API

Add a new API method that lets creators grant someone by username a specific product with a specific version.
The button in the dashboard should also be in parity with this API and get a version selection dropdown, as requested here
https://feedback.jinxxy.com/p/grant-to-user-feature-is-missing-version-option-and-some
This will be useful for giveaways. The winner could just enter their Jinxxy username using a Discord bot or a website and be automatically granted the product, without needing the creator to generate a discount code.