Add Cargo feature flag for hybrid clients to enable grpc transport

[suzmue]

As we begin developing GAPIC gRPC streaming support we are gating the code exclusively using the compiler --cfg google_cloud_unstable_gapic_streaming flag.

Cargo.toml [features] definitions are evaluated prior to rustc execution and cannot be conditionally toggled by compiler --cfg flags directly in Cargo.toml. As a result, we defer introducing a public Cargo feature flag (e.g., feature = "grpc" or feature = "unstable-stream") until the gRPC streaming API surface stabilizes.

Tasks

• Generate a public Cargo feature flag (grpc) in crate manifests once gRPC streaming APIs are ready for stabilization.
• Update generated transport templates in librarian to gate gRPC streaming types under #[cfg(all(feature = "grpc", google_cloud_unstable_gapic_streaming))].