Add interactive installed app flow

[theacodes]

Create a helper method for running an interactive installed app flow, similar to:

def credentials_flow_interactive(client_secrets_file, scopes):
    """Initiate an interactive OAuth2InstalledApp flow.

    - Display a URL for the user to visit.
    - URL displays a code for the user to copy.
    - Wait on standard input for the user to enter the provided code.
    - Exchange OAuth2 tokens.

    Args:
      client_secrets_file: The path to the client secrets JSON file.
      scopes: The list of scopes to request during the flow.
    """
    flow = google.oauth2.flow.Flow.from_client_secrets_file(
        client_secrets_file,
        scopes=scopes,
        redirect_uri='urn:ietf:wg:oauth:2.0:oob')
    auth_url, _ = flow.authorization_url(prompt='consent')
    print('Please go to this URL: %s' % auth_url)
    code = input('Enter the authorization code: ')
    flow.fetch_token(code=code)
    return flow.credentials