feat(wallets): add Turnkey signer support

[rplusq]

Summary

• Add Turnkey as a remote signer option alongside AWS KMS and GCP KMS
• Fix wallet feature flags in forge to properly gate foundry-wallets dependency

Changes

• Added alloy-signer-turnkey dependency to foundry-wallets
• Implemented Turnkey wallet CLI args and initialization logic
• Added turnkey feature flags to cast and forge
• Updated Dockerfile and Makefile to include turnkey in release builds

Bug Fix: forge wallet features

Made foundry-wallets an optional dependency in forge/Cargo.toml and updated feature definitions.

Issue: The aws-kms, gcp-kms, and turnkey features were defined but non-functional when using cargo run --features forge/turnkey. The features only worked in release builds via Makefile.

Root cause: foundry-wallets was a dev-dependency, so feature flags had no effect. Additionally, it was unconditionally compiled in the default build despite being feature-gated, causing unused dependency warnings.

Fix:

• Made foundry-wallets an optional dependency: foundry-wallets = { workspace = true, optional = true }
• Updated features to enable the dependency: turnkey = ["dep:foundry-wallets", "foundry-wallets/turnkey"]
• Added conditional import guard in lib.rs to only use the crate when features are enabled

This aligns forge with cast's wallet handling and resolves clippy warnings about unused dependencies.

Testing

# Before: --turnkey flag doesn't appear
cargo run --bin forge --features forge/turnkey -- create -h | grep turnkey
# (no output)

# After: --turnkey flag works
cargo run --bin forge --features forge/turnkey -- create -h | grep turnkey  
# --turnkey  Use Turnkey

# CI checks pass
cargo check --all                                          # ✓
cargo +nightly fmt -- --check                              # ✓
cargo +nightly clippy --all --all-targets --all-features -- -D warnings  # ✓

🤖 Generated with Claude Code

[rplusq]

Blocked by alloy-rs/alloy#2962, will update when the signer makes it into Alloy