feat: add aws-lc-rs feature as alternative crypto backend (ring stays default)

[ibigbug]

Summary

Add a new aws-lc-rs Cargo feature that lets downstream users swap the Ring crypto backend for AWS-LC. Ring remains the default — no breaking change.

Motivation

Projects like clash-rs use aws-lc-rs as their primary crypto provider across the whole binary. Without this feature, shadowquic unconditionally pulls in ring as an additional crypto library, duplicating ~500KB–1MB of crypto code (AES, curve25519, P-256 assembly) in the final binary even though those primitives are already provided by aws-lc-rs.

Changes

shadowquic/Cargo.toml

• Make ring dep optional, gated by the new ring feature (included in default)
• Add aws-lc-rs as an optional dep
• Add ring and aws-lc-rs feature flags that propagate the backend choice to all sub-crates: quinn-jls, quinn-proto-jls, rustls-jls, iroh-quinn, iroh-quinn-proto, rustls, rcgen
• Remove hardcoded rustls-ring / ring from dep features lists

Source files — replace ring:: with a feature-gated crypto_provider alias:

• src/shadowquic/quinn_wrapper/wrapper.rs — cipher suite selection + provider init
• src/sunnyquic/iroh_wrapper/wrapper.rs — same
• src/sunnyquic/dynamic_cert.rs — sign::any_supported_type
• src/sunnyquic/mod.rs — SHA256 hash via ring::digest / aws_lc_rs::digest

Usage

# Keep ring (default, unchanged behaviour)
shadowquic = { version = "0.3" }

# Opt into aws-lc-rs
shadowquic = { version = "0.3", default-features = false, features = ["shadowquic-quinn", "sunnyquic-iroh-quinn", "aws-lc-rs"] }

Verification

# default (ring) still builds
cargo check -p shadowquic

# aws-lc-rs path builds and ring dep is absent
cargo check -p shadowquic --no-default-features -F shadowquic-quinn,sunnyquic-iroh-quinn,aws-lc-rs
cargo tree  -p shadowquic --no-default-features -F shadowquic-quinn,sunnyquic-iroh-quinn,aws-lc-rs -i ring
# → empty (ring completely removed)