Description of which voice to use for speech synthesis.
Optional. The preferred gender of the voice. If not set, the
service will choose a voice based on the other parameters such
as language_code and [name][google.cloud.dialogflow.v2beta1.Vo
iceSelectionParams.name]. Note that this is only a preference,
not requirement. If a voice of the appropriate gender is not
available, the synthesizer should substitute a voice with a
different gender rather than failing the request.