input: Write a python function to encode and decode a string using Base64 encoding.
  assert encode_base64("Hello World!")=="SGVsbG8gV29ybGQh"
  assert decode_base64("SGVsbG8gV29ybGQh")=="Hello World!"
  assert encode_base64("Python is fun!")=="UHl0aG9uIGlzIGZ1biE="
  assert decode_base64("UHl0aG9uIGlzIGZ1biE=")=="Python is fun!"
output: import base64
  
  def encode_base64(input_string):
      encoded_bytes = base64.b64encode(input_string.encode('utf-8'))
      return encoded_bytes.decode('utf-8')
  
  def decode_base64(encoded_string):
      decoded_bytes = base64.b64decode(encoded_string)
      return decoded_bytes.decode('utf-8')