feat(wallet): add solana-keychain signer integration

[dev-jodee]

Summary

• Add solana-keychain-backed signer resolution for local encrypted wallets and provider-backed signer records.
• Register remote signer wallet configs for Vault, KMS, and supported signing providers.
• Keep AWS KMS support behind an optional feature so the default build does not raise the Rust MSRV.

Test Plan

• cargo check --locked --all-targets
• cargo fmt --check
• cargo test --locked
• cargo clippy --all-targets -- -D warnings
• Manual live readiness test with local encrypted wallet signer
• Manual live readiness test with HashiCorp Vault transit signer

Breaking Changes

• None