feat(puzzles): calculate product of x and y using recursion

puzzles/product_of_two_positive_integers/README.md

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+# Product of Two Positive Integers
+
+Given two numbers, find their product using recursion. In Python, we usually use the * operator to multiply two numbers, 
+but you have to use recursion to solve this challenge. 
+
+Hint:
+
+```plain
+5 * 3 = 5 + 5 + 5 = 15
+```

puzzles/product_of_two_positive_integers/__init__.py

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+def recursive_multiply(x: int, y: int) -> int:
+    """Uses recursion to find the product of x and y
+    Args:
+        x (int): first number
+        y (int): second number
+    Returns:
+        int: result of multiplication
+    """
+    # this cuts down the recursive calls
+    if x < y:
+        return recursive_multiply(y, x)
+    if y == 0:
+        return 0
+    return x + recursive_multiply(x, y - 1)

puzzles/product_of_two_positive_integers/test_recursive_multiply.py

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+import unittest
+from . import recursive_multiply
+
+
+class RecursiveMultiplyTestCase(unittest.TestCase):
+    def test_1(self):
+        """should return 15 from 5 and 3"""
+        x = 5
+        y = 3
+        expected = x*y
+        actual = recursive_multiply(x, y)
+        self.assertEqual(expected, actual)
+
+    def test_2(self):
+        """should return 1000000 from 500 and 2000"""
+        x = 500
+        y = 2000
+        expected = x*y
+        actual = recursive_multiply(x, y)
+        self.assertEqual(expected, actual)
+
+if __name__ == '__main__':
+    unittest.main()