ci: add lint checks and remove dead feature flag

[swaroop-osec]

• Add CI workflow with cargo fmt, clippy, cargo-sort, cargo-machete checks, and cargo test on push/PR to master
• Add rust-toolchain.toml pinning Rust 1.93 (matches Dockerfile)
• Remove dead use-external-pdas feature flag and related gated imports that we are not using anywhere in the codebase currently.