[V1] Use msgpack for core request serialization

[njhill]

We were using pickle rather than msgpack for serializing the messages from the front-end process to the engine, because msgpack doesn't natively support tensors and these need to be included for multimodal requests.

These can be handled easily though via custom msgpack extension. This should mean more efficient serialization.

This PR also simplifies handling of different request types.

[github-actions]

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