ci: Publish Rust SDK crates to crates.io via GitHub Actions

[zeljkoX]

Add GitHub Actions workflow to publish Guardian Rust crates to crates.io in dependency order.

Crates to publish:

• guardian-shared
• guardian-client
• miden-multisig-client
• miden-confidential-contracts

Auth: Prefer crates.io OIDC trusted publishing. Fallback: CARGO_REGISTRY_TOKEN in release environment.

Requirements:

• cargo publish per crate in order. Wait for indexing between publishes
• Trigger on release/workflow_dispatch with version validation
• environment: release, harden-runner, pinned SHAs, repo owner guard
• Install: protobuf-compiler, libpq-dev, Rust 1.93.0
• cargo publish --dry-run on PRs touching workflow