End-user based API keys

I got the request from one of my paying end-users to get an API key so he can integrate the app in his business software.

Since there is only a single key for the whole app I cannot give that to him because I’d lose all control. It would be create if there was an additional authentication layer, so end-users might have to use their email and password to use an API and there would need to be a mechanism to manage this, so only white-listed or subscribed users can use this.