feat: [TextToSpeech] Support promptable voices by specifying a model name and a prompt (#8542)

* feat: Support promptable voices by specifying a model name and a prompt
feat: Add enum value M4A to enum AudioEncoding
docs: A comment for enum value `AUDIO_ENCODING_UNSPECIFIED` in enum `AudioEncoding` is changed

PiperOrigin-RevId: 799573824

Source-Link: googleapis/googleapis@a92cee3

Source-Link: googleapis/googleapis-gen@d182858
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* 🦉 Updates from OwlBot post-processor

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* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

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Author: gcf-owl-bot[bot]