refactor: update the code as the laravel best practices

[huzaifaarain]

Refactor the code as per Laravel best practices

After building couple of applications using this template I came across couple of issues and I have tried to address those in the PR to the best of my knowledge.

Using env directly in the service provider is not recommended. All env values should be resolved via config.

Decoupled shopify related business logic from AppServiceProvider, extract the logic into its own service provider ShopifyServiceProvider

Implementing business logic directly into the web.php is not recommended. I have also organized the web.php file by moving the business logic to its own namespace and relevant controllers.

[huzaifaarain]

I have signed the CLA!