Fix billable_id

[Emericanec]

https://github.com/lmsqueezy/laravel/issues/21

In my project I have uuid for billable entities. So seems it better to use varchar in migrations

In WebhookController it's already int|string type. I tasted it on my project and it works

[driesvints]

Thanks. I do think using an integer is the vastly majority of the use cases. For your use case, I've written some docs to overwrite migrations: https://github.com/lmsqueezy/laravel/commit/60ceab400630d39c5f628d34c5c7838286e4d5db

[Emericanec]

Thanks!