Allow developers to set own license keys to their apps and games

[AlmasB]

Please discuss the feature ideas on Gitter first. Then please describe the feature
Developers may wish to sell their apps and games which can only be played if the end-user purchased the license key from the developers.

At the moment, it's unclear how this feature is going to work. Developers will need to have the ability to manage their own license keys, so it seems this service will need to talk to developers' servers or somehow obtain the validity of the entered key.

[CharlyZhu]

I would like to give this a try, I could create an API for user to implement and hook up with their own choice of licensing and key systems, then add the functionality to prevent the app from running if the license or the key is invalid.

[AlmasB]

@CharlyZhu go for it, feel free to post your API design here and how it would hook into the FXGL lifecycle