input: Write a python function to find the sum of all values in a binary tree.
  assert sum_tree_values(TreeNode("root", 10, TreeNode("left", 5), TreeNode("right", 15))) == 30
  assert sum_tree_values(TreeNode("root", 1, TreeNode("left", 2, TreeNode("left.left", 3)), TreeNode("right", 4))) == 10
  assert sum_tree_values(TreeNode("root", 0)) == 0
output: class TreeNode:
      def __init__(self, key, value=0, left=None, right=None):
          self.key = key
          self.value = value
          self.left = left
          self.right = right
  
  def sum_tree_values(node):
      if node is None:
          return 0
      return node.value + sum_tree_values(node.left) + sum_tree_values(node.right)