feat(tts): add support for streaming mode

[mudler]

Description

This PR adds tts streaming support. It currently enables it for voxcpm (not all backends supports it)

To enable streaming, add "stream": true to the request:

curl http://localhost:8080/tts -H "Content-Type: application/json" -d '{
  "input": "Hello world, this is a streaming test",
  "model": "voxcpm",
  "stream": true
}' | aplay

Model config:

name: voxcpm
known_usecases:
- tts
backend: voxcpm
parameters:
  model: openbmb/VoxCPM1.5

Notes for Reviewers

• This PR also adds an e2e mocked tests suite (with a fake grpc backend) to test e2e all endpoints (http client -> http server -> grpc backend). This is particularly useful for quick tests against the routes, avoiding the full load of models
• This PR also re-enables running e2e tests as part of the CI jobs

Signed commits

• Yes, I signed my commits.

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