Gender of the voice as described in SSML voice
element <https://www.w3.org/TR/speech-synthesis11/#edef_voice>__.
Values:
SSML_VOICE_GENDER_UNSPECIFIED (0):
An unspecified gender.
In VoiceSelectionParams, this means that the
client doesn't care which gender the selected
voice will have. In the Voice field of
ListVoicesResponse, this may mean that the voice
doesn't fit any of the other categories in this
enum, or that the gender of the voice isn't
known.
MALE (1):
A male voice.
FEMALE (2):
A female voice.
NEUTRAL (3):
A gender-neutral voice. This voice is not yet
supported.
