Allow Extending Subscription and Billable model

[vaishnavmhetre]

My base model design pattern contains usage of UUID as the base identifier/primary key. Unfortunately, I can't extend the Trait to the Subscription model to follow the design pattern.

This might be a tedious task to implement (Update all Subsciption model imports), but rather would be useful for scalable usability.

[msonowal]

Feel free to send a PR @vaishnavmhetre