feat(tts): add Elevenlabs and OpenAI TTS compatibility layer (#1834)

* feat(elevenlabs): map elevenlabs API support to TTS

This allows elevenlabs Clients to work automatically with LocalAI by
supporting the elevenlabs API.

The elevenlabs server endpoint is implemented such as it is wired to the
TTS endpoints.

Fixes: https://github.com/mudler/LocalAI/issues/1809

* feat(openai/tts): compat layer with openai tts

Fixes: #1276

* fix: adapt tts CLI

backend/backend.proto

@@ -191,6 +191,7 @@ message TTSRequest {
   string text = 1;
   string model = 2;
   string dst = 3;
+  string voice = 4;
 }
 
 message TokenizationResponse {