Builds a client config dictionary used in the OAuth 2.0 flow.

def Build(self):
    """Builds a client config dictionary used in the OAuth 2.0 flow."""
    if all((self.client_type, self.client_id, self.client_secret,
            self.auth_uri, self.token_uri)):
      client_config = {
          self.client_type: {
              'client_id': self.client_id,
              'client_secret': self.client_secret,
              'auth_uri': self.auth_uri,
              'token_uri': self.token_uri
          }
      }
    else:
      raise ValueError('Required field is missing.')

    return client_config