Adding support to route a request to a specific engine instance

[dumb0002]

This PR adds support to route a request to a specific engine instance., e.g., vLLM v1 sleep and wake_up for a specific vLLM engine.

To achieve that we added the following features:
a) Add a end route to return the list of available engines and their serving models: /engines, Methods: POST - For example:

$ curl -o- http://localhost:30080/engines | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   225  100   225    0     0   1303      0 --:--:-- --:--:-- --:--:--  1308
[
  {
    "engine_id": "b1450413-a704-56d6-a95f-ea0e4bb21ada",
    "serving_model": "facebook/opt-125m",
    "created": 1747669342
  },
  {
    "engine_id": "03dc3c14-fea9-527b-a715-da92ac7ebce6",
    "serving_model": "openai-community/gpt2",
    "created": 1747669342
  }
]

b) Add a route to send a inference to a specific engine instance based on the ID returned in (a) above. The engine ID is provided using a query string:

$ curl -X POST http://localhost:30080/v1/completions?id=b1450413-a704-56d6-a95f-ea0e4bb21ada \
>   -H "Content-Type: application/json" \
>   -d '{
>     "model": "facebook/opt-125m",
>     "prompt": "The capital of France is,",
>     "max_tokens": 10
>   }' \
>   | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   488  100   385  100   103   1738    465 --:--:-- --:--:-- --:--:--  2208
{
  "id": "cmpl-8e9948646f0d45e3b26291f944430f3f",
  "object": "text_completion",
  "created": 1747669434,
  "model": "facebook/opt-125m",
  "choices": [
    {
      "index": 0,
      "text": " surprisingly, a French swim pool. Seriously, no",
      "logprobs": null,
      "finish_reason": "length",
      "stop_reason": null,
      "prompt_logprobs": null
    }
  ],
  "usage": {
    "prompt_tokens": 7,
    "total_tokens": 17,
    "completion_tokens": 10,
    "prompt_tokens_details": null
  }
}

First PR for FIX to issue #391

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[max-wittig]

Hi! Would it be possible to add some tests to this feature?

[ahinsutime]

Will it be better to check endpoint.Id (engine_id) instead of model_name?
If there are multiple replicas for the same model, only one of them will be considered.

[dumb0002]

Yes! replaced with endpoint.Id

[YuhanLiu11]

@dumb0002 Thanks! This is great! can you resolve the conflict?

[dumb0002]

@dumb0002 Thanks! This is great! can you resolve the conflict?

Done!

[YuhanLiu11]

LGTM! Thanks for your contribution!