Allow to pass `default_response_class` e.g. `ORJSONResponse` to LitServe server

Nobody has claimed this yet.

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## 🚀 Feature

Allow to pass `default_response_class` e.g. `ORJSONResponse` downstream to the underlying `FastAPI` app.

### Motivation

I really love LitServe, but one thing that I feel is missing is the ability to set the default response class.
After some research and informal experiments with the [orjson project](https://github.com/ijl/orjson) I experienced certain efficiency gains that I'd like to carry over to my LitServe projects.
Do you think it would be worth it to allow for such downstream param passing? If so, I'd be happy to submit a PR :)

### Pitch

Passing custom response classes to the `FastAPI` like `ORJSONResponse` might be a convenient feature with potential of speeding up the serialization and deserialization of JSON.

### Additional context

This approach is also referenced in `FastAPI` docstrings:
https://github.com/fastapi/fastapi/blob/b0215699137c41a96c57f650640cb9db1c25314a/fastapi/applications.py#L369

Start by locating where LitServe constructs its underlying FastAPI app and compare the `default_response_class` parameter in FastAPI's linked `applications.py`. Done means the server accepts a custom response class such as `ORJSONResponse` and passes it to the underlying FastAPI app.

Written by the indexing model from the issue text.