Implement AuthHandler.sign_string

[lalinsky]

OAuth2ServiceAccountAuth should implement the sign_string method, so that generate_url can work. The code below works for me, but it should be abstracted and I'm not sure what are all the auth methods that the plugin supports:

  def sign_string(self, string_to_sign):
    import base64
    from oauth2client.crypt import Signer
    signer = Signer.from_string(self.oauth2_client._private_key)
    return base64.b64encode(signer.sign(string_to_sign))

[bharris47]

Any plans to implement this? generate_url is broken even when generating an unsigned url.