Expand `createEmptyCart` mutation

[naydav]

• Need to add optional input parameter “cart_id” (like predefined id) to “createEmptyCart” mutation
  https://github.com/magento/graphql-ce/blob/2.3-develop/app/code/Magento/QuoteGraphQl/etc/schema.graphqls#L9
• Validation should be added
• API functional tests should be added

Examples:
https://github.com/magento/graphql-ce/blob/2.3-develop/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Customer/CreateEmptyCartTest.php

https://github.com/magento/graphql-ce/blob/2.3-develop/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Guest/CreateEmptyCartTest.php

[anzin]

@naydav, can you help me ?
How to validate cart_id? The length should be equal to 32, as in the standard implementation: http://joxi.ru/Dr8M0GMtozdgEm ? Or can it be smaller?