Add Rust graph toolchain support

You can now build Rust projects in Once using the new Rust toolchain targets. First-party code uses rust_library, rust_binary, and rust_proc_macro, while resolved Cargo packages are lowered into rust_crate targets. External dependencies are managed by a cacheable cargo_dependencies target that runs cargo metadata –locked and keeps Cargo.toml and Cargo.lock as the source of truth. Rust targets also support Apple static libraries and Android native libraries through crate_type and Android linker options.