feat: Use gRPC APIs for Gemini + xAI

[dxvid-pts]

Feature Request

Use the gRPC API for the Rig model providers that expose gRPC APIs (currently Google (Gemini Direct API) and xAI)

Docs + proto sources:

• Google Gemini Direct API (REST/gRPC) docs:
  • https://ai.google.dev/gemini-api/docs/partner-integration
• Google proto source (Gemini / Generative Language API):
  • https://github.com/googleapis/googleapis/blob/master/google/ai/generativelanguage/v1beta/generative_service.proto
• xAI gRPC docs + proto sources:
  • https://docs.x.ai/docs/grpc-reference
  • https://github.com/xai-org/xai-proto
  • https://github.com/xai-org/xai-proto/blob/736b835b0c0dd93698664732daad49f87a2fbc6f/proto/xai/api/v1/chat.proto

Motivation

• Protobuf is strongly typed, more performant to de/serialize than JSON, smaller than JSON, easy to generate via the tonic crate and backwards compatible

Proposal

• Use the gRPC backend for the existing gemini and xai providers instead of the current REST backend
• Implementation outline:
  • Use tonic + prost (or equivalent) to generate clients from the upstream protos.
  • Google:
    • Generate a client for google.ai.generativelanguage.v1beta.GenerativeService from the googleapis proto.
    • Attach x-goog-api-key (and optionally x-goog-api-client for identification) metadata headers.
  • xAI:
    • Generate clients from xai-org/xai-proto and connect to api.x.ai with Authorization: Bearer <API_KEY> metadata.
  • Map Rig’s existing request/response types to/from the proto messages; preserve existing streaming APIs (stream_prompt, etc.) by adapting gRPC streaming responses into Rig’s stream types.

[linear]

RIG-1116

[joshua-mo-143]

This would likely pull in quite a lot of new dependencies so I don't think this would be in scope for rig(-core) but would be happy to have this as a feature gated side crate!