feat: Course Upgrade Option with IAP

[saeedbashir]

This PR adds course upgrade option with IAP. At the moment, support is being added for non-consumable products.

###Config:

Local Config:
ECOMMERCE_URL: 'https://ecommerce.example.com' // e-commerce base URL for the course upgrade application

The feature is configurable via server config.

{
  "iap_config": {
    "enabled": true,
    "restore_enabled": false,
    "ios_disabled_versions": "[]"
  },
  "value_prop_enabled": true // to hide/show upgrade information messages.
}

video.mp4

Note: The work is in progress and can have some issues, but the architecture is almost final and will not change much.

[saeedbashir]

It's decided that this change will not go upstream so closing it