Add the ability to invoke an external key generator

Please add the ability to invoke an external key generator, and add support for multi line keys.

The generator should be callable by URL with placeholders for the generator name, amount of bought licenses if multiple of the same type ware bought as well as for the other customizable parameters.

The use case here not to use the plugins facilities for activation, deactivation and license validation, but just deploy externally generated license data which would be handles by other 3rd party mechanisms in the licensed software.

I have a ugly fork that does that partially based on an old plugin version so i can post it if there is interest.