feat: Add an option for specifying model name

[vrdn-23]

What does this PR do?

This PR introduces a new CLI argument, --served-model-name, which allows users to specify a custom model name to be returned in responses from the OpenAI-compatible endpoint.

This is particularly useful in scenarios where the model is loaded from a local path (e.g., /data/model) and does not have an inherent name associated with it. By setting --served-model-name, users can override the default model identifier (which might be a generic or filesystem-based value) and provide a more descriptive or meaningful name in the API response. This helps improve clarity and consistency, especially when integrating with clients or tools that rely on the model field in the response for tracking or routing purposes.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@Narsil @alvarobartt @kozistr

I have tested this to the best of my ability, but I'm not sure if I did the gRPC bits correctly, so if someone could help verify that, that would be great!