Synthesizes speech and plays back the synthesized audio to the
caller in Telephony Gateway.
Telephony Gateway takes the synthesizer settings from
DetectIntentResponse.output_audio_config which can either be set
at request-level or can come from the agent-level synthesizer
This message has oneof_ fields (mutually exclusive fields).
For each oneof, at most one member field can be set at the same time.
Setting any member of the oneof automatically clears all other