input: Write a python function to encode a string using base64 encoding.
  assert encode_string("hello")== "aGVsbG8="
  assert encode_string("world")== "d29ybGQ="
  assert encode_string("base64")== "YmFzZTY0"
output: def encode_string(s):
      import base64
      return base64.b64encode(s.encode()).decode()