**What**
On cart creation:
- If region only has one country -> create cart with country code
On cart updates:
- If shipping address country code is provided in input ->
- If cart region doesn't include that country -> throw
- If cart includes the country -> update shipping address
- If region is provided in input and is different from the one currently on the cart ->
- If there is a shipping address on the cart -> clear the address
- If the region only has one country -> set country code of address
- If there is not a shipping address on the cart ->
- If the region only has one country -> set country code of address
Closes CC-545
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