input: Write a python function to pad a string with a given character to a specified length.
  assert pad_string("cat", "0", 5) == "00cat"
  assert pad_string("dog", "*", 6) == "***dog"
  assert pad_string("hello", "x", 10) == "xxxxxhello"
output: def pad_string(s, char, length):
      return char * (length - len(s)) + s