feat: Use gRPC APIs for Gemini + xAI

## 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 ` 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.

The issue requires generating gRPC clients from the provided proto files for Google Gemini and xAI using the `tonic` and `prost` crates. Start by examining the existing `gemini` and `xai` provider implementations in the codebase to understand the current REST API integration. Then, map Rig's request/response types to the generated proto messages, ensuring streaming support is maintained. Testing will involve verifying the new gRPC connections work with the respective authentication headers.

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