[Misc] Minimum requirements for SageMaker compatibility

[nathan-az]

Fixes #11557

Implements /ping and /invocations, and creates an alternate dockerfile, identical to vllm-openai but with entrypoint setting port to 8080.

Since the OpenAI server is more "production-ready" we use this functionality and its handlers as the base.

Considerations:

Dockerfile

The Dockerfile order has changed, defining the vllm-sagemaker image first, then building from that for vllm-openai.

This avoids repetition of the additional dependencies, and still defines vllm-openai last, so that it is the default for docker build. If we don't like using vllm-sagemaker as the base for vllm-openai we can simply repeat the additional requirements between both, and revert to from vllm-base as vllm-openai.

Routing

• The app state has slightly changed to include the model task to aid with inferring the correct task
• We lose the FastAPI casting of the incoming request to the correct Pydantic model, so we explicitly do this with model_validate
• We use whether messages is in the request to determine whether it is a chat input

Note that these changes make no changes to other images or APIs. IMO it should be ok to integrate them for the purpose of expanding to SageMaker use cases, without offering the full flexibility of being able to make requests to all the endpoints.

I have tested the new endpoints locally. I will be able to test building and deploying on SageMaker some time in the next couple of weeks, but welcome feedback.

[github-actions]

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[nathan-az]

Will remake this to clean the git history and sign properly