Provide example code for texttospeech streaming synthesis

[chrbsg]

There is no existing sample code that demonstrates use of StreamingSynthesize for input-streaming TTS. It would be great if this could be available at https://cloud.google.com/text-to-speech/docs/samples/tts-quickstart

Code should document that there is no way to configure audio output. Only LINEAR16 audio with a sample rate of 24000Hz is available.

Code should document that most text-to-speech voices will not work - only Journey voices are supported by streaming synthesis.

(originally reported as google-cloud-go issue #10955)