TTS API improvements (#2308)

* update doc on COQUI_LANGUAGE env variable

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* return errors from tts gRPC backend

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* handle speaker_id and language in coqui TTS backend

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* TTS endpoint: add optional language paramter

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* tts fix: empty language string breaks non-multilingual models

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* allow tts param definition in config file

- consolidate TTS options under `tts` config entry

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* tts: update doc

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Co-authored-by: Ettore Di Giacinto <mudler@users.noreply.github.com>