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This PR adds support for reading the `OAI_ENABLED` env var which will
changes the function called when the `/invocations` is called.

If `OAI_ENABLED=true` the `chat_completions` method is used otherwise it
defaults to `compat_generate`.

example running the router
```bash
OAI_ENABLED=true \
  cargo run -- \
  --tokenizer-name mistralai/Mistral-7B-Instruct-v0.2
```

example request
```bash
curl localhost:3000/invocations \
    -X POST \
    -d '{ "model": "tgi", "messages": [ { "role": "user", "content": "What is the IP address of the Google DNS servers?" } ], "stream": false, "max_tokens": 20, "logprobs": true, "seed": 0 }' \
    -H 'Content-Type: application/json' | jq 
```

**please let me know if any naming changes are needed or if any other
routes need similar functionality.