Add support for expiring offline access tokens to shopify-app-express

[dansundy]

Overview

It currently doesn't seem possible to request expiring offline access tokens when using the @shopify/shopify-app-express. As of April 1 this is now required, so not being able to request expiring offline tokens is problematic for any new apps using this library.

Could we either have the future flag:

shopifyApp({
  api: { ... },
  future: {
    expiringOfflineAccessTokens: true
  }
});

Or even just flag on the existing auth object?

shopifyApp({
  api: { ... },
  auth: {
    path: '/api/auth',
    callbackPath: '/api/auth/callback'
    expiring: true
  }
});