Introducing wasi-grpc for Spin

Spin 3.4 introduces outbound HTTP/2 support, enabling WebAssembly components to act as first-class gRPC clients. The new wasi-grpc crate allows Spin applications to directly communicate with gRPC services without requiring sidecars or proxy services. A complete tutorial demonstrates building a Rust-based Spin component that connects to a gRPC service using tonic and the wasi-grpc crate, including setup, configuration, and implementation details.