input: Write a python function to convert given text to speech and save it as an MP3 file using Google Cloud Text-to-Speech API.
  assert os.path.exists('output.mp3') == False
  convert_text_to_speech('Hello World!', 'output.mp3')
  assert os.path.exists('output.mp3') == True
output: def convert_text_to_speech(text, output_filename):
      from google.cloud import texttospeech
      import os
      
      # Initialize the Text-to-Speech client
      client = texttospeech.TextToSpeechClient()
  
      # Set the text input to be synthesized
      synthesis_input = texttospeech.SynthesisInput(text=text)
  
      # Build the voice request, select the language code ("en-US") and the ssml voice gender
      voice = texttospeech.VoiceSelectionParams(
          language_code="en-US",
          ssml_gender=texttospeech.SsmlVoiceGender.NEUTRAL
      )
  
      # Select the type of audio file you want returned
      audio_config = texttospeech.AudioConfig(
          audio_encoding=texttospeech.AudioEncoding.MP3
      )
  
      # Perform the text-to-speech request
      response = client.synthesize_speech(
          input=synthesis_input, voice=voice, audio_config=audio_config
      )
  
      # Save the response audio to the output file
      with open(output_filename, "wb") as out:
          out.write(response.audio_content)