feat(wfe-rustlang): add Rust toolchain step executors

New crate providing cargo and rustup step types for WFE workflows:

Cargo steps: build, test, check, clippy, fmt, doc, publish
Rustup steps: rust-install, rustup-toolchain, rustup-component, rustup-target

Shared CargoConfig base with toolchain, package, features, release,
target, profile, extra_args, env, working_dir, and timeout support.
Toolchain override via rustup run for any cargo command.

wfe-rustlang/src/cargo/mod.rs

@@ -0,0 +1,5 @@
+pub mod config;
+pub mod step;
+
+pub use config::{CargoCommand, CargoConfig};
+pub use step::CargoStep;