Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.

Note: You must not use any built-in BigInteger library or convert the inputs to integer directly.

Input: num1 = "2", num2 = "3"
Output: "6"

Input: num1 = "123", num2 = "456"
Output: "56088"