refactor: restructure Rust plugins as independent crates

[lucarlig]

Closes #2730

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Remove workspace-level Cargo.toml and make each Rust plugin a standalone crate with its own build configuration. This simplifies plugin development by eliminating workspace dependencies and allowing plugins to be built, tested, and distributed independently.

Each plugin now has:

• Its own Cargo.toml with independent dependencies
• Dedicated Makefile for standalone builds
• Self-contained pyproject.toml for Python bindings
• Updated documentation reflecting the new structure

This change reduces complexity and makes it easier to add new Rust plugins without affecting existing ones.

[lucarlig]

@crivetimihai ready for review

[crivetimihai]

Review & Rebase

Rebased onto main (was 44 commits behind) and added a fix commit to address broken references from the restructuring.

What was fixed

The original commit correctly restructured pii_filter as an independent crate but left many dependent references pointing at the old workspace structure. The fix commit updates:

File	Issue
.github/workflows/rust-plugins.yml	release job referenced deleted python-integration job; make targets don't exist for all plugins (e.g. encoded_exfil_detection has no Makefile) — switched to direct cargo commands
Containerfile, Containerfile.lite, Containerfile.scratch	Builder stage assumed workspace-level Cargo.toml (WORKDIR /build/plugins_rust + maturin build); wheel glob used old package name mcpgateway_rust-*; verification imported from plugins_rust
Makefile	18+ rust targets used cd plugins_rust && cargo/maturin which requires the deleted workspace Cargo.toml; rust-verify imported old module name; rust-bench-compare pointed to nonexistent path
MANIFEST.in	Referenced deleted plugins_rust/Makefile
docs/docs/using/plugins/rust-plugins.md	Troubleshooting section still referenced from plugins_rust import, mcpgateway-rust package name, bare cd plugins_rust commands
plugins_rust/README.md	Used generic placeholders that didn't match actual code; referenced nonexistent make install-all
plugins_rust/pii_filter/src/detector.rs	Doc comment still referenced from plugins_rust import

Performance claim

Reverted "5-100x" back to "5-10x" — the benchmark data (which was removed from the docs) showed 6.7x-8.3x speedups.

Commits

dc93645 fix: update references for independent Rust plugin crate structure
58e6ebb refactor: restructure Rust plugins as independent crates