App support

[qw-in]

Since Shopify has EOL'd the old private apps, I think it would be convenient for SpyLib to have some concept of an App

I don't think it has to be super complex either. It could really just provide convenience wrappers around existing functionality. Below is a simple example:

# Verifying webhooks without an `App`

@router.post('/webhooks')
def handle_webhook():
    verify_webhook(body, header, shopify_api_secret_key)


# Verifying webhooks with an `App`

app = App(shopify_api_key='', shopify_api_secret_key='')

@router.post('/webhooks')
def handle_webhook():
    app.verify_webhook(body, header)

The app could also contain more global configuration such as api version, required scopes, public_url, etc.