Added GraphQL support for Replicate Cart in Store endpoint

You can now replicate any Cart or Order in a given Store to a new Cart bound to the same Store using the GraphQL API.

• [GraphQL API] Added support for Replicate a Cart in a Store endpoint.

Introduced the following changes to the GraphQL schema (in SDL format):

extend type Mutation {
  replicateCart(key: String!, reference: ReferenceInput!): Cart
}