Split "Update a Cart" into two separate API points

[Deroswent]

The Update a Cart API endpoint and updateCartWorkflow now performs two functions:

1. updates data in the cart, such as billing_address, shipping_address, additional_data
2. it does a “cart reset”: this will cancel the previously selected shipping and payment methods and the buyer will have to select them again.

It is a very strange solution to combine these two functions in one endpoint. Besides, it is impossible to somehow avoid “cart reset”.

I suggest splitting this endpoint into two separate endpoints. Because it's not always necessary to re-specify the shipping or payment method if the user just decided to change the address.