input: Write a function to create a virtual assistant using Python and Dialogflow to answer a basic set of questions. The function should take a user's input, send it to the Dialogflow agent, and print the assistant's response. The function should use the specified project ID for Dialogflow and an example session ID.
  assert virtual_assistant('What is the weather today?')
  assert virtual_assistant('Who is the CEO of Google?')
  assert virtual_assistant('How do I cook pasta?')
output: 
  def virtual_assistant(user_input):
      import dialogflow_v2 as dialogflow
      import os
  
      os.environ["GOOGLE_APPLICATION_CREDENTIALS"] = "path_to_your_credentials_file.json"
      project_id = "your_project_id"
      session_id = "example_session_id"
  
      session_client = dialogflow.SessionsClient()
      session = session_client.session_path(project_id, session_id)
  
      text_input = dialogflow.types.TextInput(text=user_input, language_code="en")
      query_input = dialogflow.types.QueryInput(text=text_input)
  
      response = session_client.detect_intent(session=session, query_input=query_input)
      print(response.query_result.fulfillment_text)
  