Added key to Cart

Additionally to identifying Carts by their id, it is now possible to use key as a user-defined identifier for the list of operations below. This gives you more flexibility when referencing Carts from commercetools-external systems.

• [API] Added optional key field to Cart and CartDraft.
• [API] Added Get a Cart by Key method.
• [API] Added Update a Cart by Key method.
• [API] Added Delete a Cart by Key method.
• [API] Added Get a Cart in a Store by Key method.
• [API] Added Update a Cart in a Store by Key method.
• [API] Added Delete a Cart in a Store by Key method.
• [API] Added SetKey update action.
• [API] Added Replicate a Cart by Key method.
• [API] Added cart field to OrderFromCartDraft.
• [API] Deprecated the id field in OrderFromCartDraft in favor of the cart field.