text: Title: Add ability to specify extra during checkout
  Body: **Is your feature request related to a problem? Please describe.**
  Saving extra data on order (eg. "phone number") during the checkout requires an additional POST request to `/api/basket/extra/`.
  
  **Describe the solution you'd like**
  When sending a POST request to `/api/checkout/` add an ability to send `extra` data in a payload directly.
  
  **Describe alternatives you've considered**
  `-`
  
  **Additional context**
  Validation for extra data should be enforced here as well (#1).
  