Merge fastapi server repo

[lazzarello]

Changes

• Merges the project at xtts-streaming-server into a new directory
• Adds a Dockerfile to the demo for easy deployment
• Adds an environment variable TTS_API_ENDPOINT to the demo to change the URL for the GPU backend when not running on localhost
• Adds a systemd unit to start the server on boot

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[lazzarello]

sorry, wrong branch. closing.