input: Write a python function to filter digits from a string.
  assert filter_string("abc123") == "123"
  assert filter_string("4x4s4s4s4s4s") == "44444"
  assert filter_string("no_digits_here") == ""
output: def filter_string(s: str) -> str:
      return ''.join(filter(str.isdigit, s))