[FEATURE-REQUEST] - Add Proto Messages for Response API

[matthew-bram]

This feature request is to add the response APIs found here https://api.x.ai/docs/#/ as protocol buffers in the xai-proto repo.

Why this matters:

1. Helps engineers reduce marshalling/unmarshalling concerns when integrating with the API.
2. Improved performance and reduction of network egress costs out of cloud service providers.
3. Strict type safety without needing external schemas.

I'm hoping that the answer is not: "Just use our python SDK." For those of us allergic to interpreted languages and adverse to running other's sdk code, it would would be able to integrate directly with the gRPC gateway.

Thank you for your consideration!