Add support for billing for apps

[paulomarg]

What this PR does

Partners might want to charge for their apps, but this gem currently doesn't really add any specific support for that scenario.

This PR adds that support by appending a concern to Authenticated to check if there is an active payment for the merchant, or requesting one if there isn't, and redirecting the merchant to it.

If the new billing configuration setting isn't present, this does nothing.

Reviewer's guide to testing

This was mostly tested by unit tests, and by activating the configuration in a new app.

Things to focus on

1. Does this break any pre-existing flows? As in, are there cases where we might try to apply billing when we shouldn't?
2. Is the test coverage good?

Checklist

Before submitting the PR, please consider if any of the following are needed:

• Update CHANGELOG.md if the changes would impact users

[paulomarg]

This was making some tests flaky - webmock is already blocking all requests so we don't need to do this here.

[hannachen]

Thanks for adding this! I was able to 🎩 and do a smoke test with the existing flow without encountering issues.

I did encounter an interesting flow, where if the app was initially installed without billing, but later introduced to the app. The billing screen won't show up until the next merchant log in/session, is this expected behaviour?

[paulomarg]

The billing screen won't show up until the next merchant log in/session, is this expected behaviour?

Yes - because we need to query Shopify for the billing status for a merchant, we need to have access to a token, which only happens on authenticated requests, or when coming back from OAuth. Those are the two points where we check for billing.

I'm going to merge the PR but we can fix forward if there are any other scenarios we want to cover!

[immakdas]

The main scenario is not covered so it is useless

1. merchant user clicks to Install the app
   
2. on the payment page customer click cancel
   
3. as a result App is installed but not paid and the user can Open it for free
   

tested with the shopify_app 20.0.2
I believe the Trial scenario is also not covered

config.billing = ShopifyApp::BillingConfiguration.new(
     charge_name: "My app billing charge",
     amount: 5,
     interval: ShopifyApp::BillingConfiguration::INTERVAL_EVERY_30_DAYS,
     currency_code: "USD",
     trial: 1,
  )```

return
`gems/shopify_app-20.0.2/lib/shopify_app/configuration.rb:119:in `initialize': unknown keyword: :trial (ArgumentError)`