Add Rust language support for PluMA plugins

[quinnjr]

Summary

• Add Rust language support allowing PluMA plugins to be written in Rust
• Implement Rust FFI loading mechanism in Rust.h/Rust.cxx
• Add RustIO.rs module providing FFI bindings for Rust plugins
• Add Cargo.toml for the pluma crate (Rust plugin interface)
• Update PluginManager to register Rust language handler
• Update SConstruct with --with-rust and --build-rust-plugins options
• Add RustPluginGenerator for PluGen --lang=rust scaffolding
• Update PluGen README with Rust plugin generation documentation
• Update CHANGELOG with v2.1.0 Rust support details

Test plan

• Build PluMA with --with-rust option enabled
• Generate a Rust plugin using PluGen --lang=rust
• Verify Rust plugin compiles and loads correctly
• Test plugin input/output operations via RustIO bindings