Introduced 'cart' field on QuoteRequest

With the introduction of the cart field on QuoteRequest, you can now know the Cart from which a Quote was requested.

Changes:

• [API] Added the cart field to QuoteRequest.
• [GraphQL API] Changed the QuoteRequest type:
  • Added the cartRef field to the QuoteRequest type.

The following changes were introduced in terms of GraphQL SDL:

extend type QuoteRequest {
 cartRef: Reference
}