[gRPC] gRPC-in-gRPC-out for Generate API

Change EPP to implement gRPC-to-gRPC for Generate API (Non-streaming & Streaming)
* Import and manage vLLM gRPC [proto](https://github.com/vllm-project/vllm/blob/main/vllm/grpc/vllm_engine.proto) definitions (e.g., in pkg/epp/proto).
* Implement gRPC message pass-through/parsing for the Generate API.
* Ensure support for both unary and server-side streaming gRPC calls.