Added Cart freeze option

We have added the option to freeze Carts during the purchasing journey of your Customers. With this, usual price and discount updates are primarily not applied on frozen Carts. This new feature is useful if you want to prevent Carts from being modified by changes affecting prices until the order is placed. Inventory is not reserved by freezing a Cart.

Changes:

• [API] Added FreezeCart and UnfreezeCart update actions to Carts API.
• [API] Added Frozen field to CartState.
• [GraphQL API] Added the following types to the GraphQL schema: FreezeCart, UnfreezeCart.
• [GraphQL API] Added input fields freezeCart and unfreezeCart to the CartUpdateAction type.
• [GraphQL API] Added Frozen value to the CartState enum type.

The following changes were introduced in terms of GraphQL SDL:

extend input CartUpdateAction {
  freezeCart: FreezeCart
  unfreezeCart: UnfreezeCart
}

extend enum CartState {
  "The cart is frozen. The cart is ready for checkout and a few update actions are not allowed."
  Frozen
}

input FreezeCart {
  dummy: String
}

input UnfreezeCart {
  dummy: String
}