[WIP] Copilot Request

[Copilot]

• Add the modules/sequoia-sq registry metadata and versioned module files
• Pin the upstream sequoia-sq 1.4.0 source archive with verified integrity and overlay hashes
• Add the module presubmit and versioned overlay for a Rust-based @sequoia-sq//:sq target
• Validate the registry integrity and selection checks

This module follows the source-build path using rules_rust and crate_universe, selecting the pure-Rust Sequoia crypto backend (crypto-rust) so the sq binary can be built without shipping custom, non-verifiable upstream binaries. The registry entry pins sequoia-sq at 1.4.0.envoy and keeps the envoyproxy/toolshed bazel/pgp/extensions.bzl integration point intact until it can be simplified to consume @sequoia-sq//:sq via bazel_dep instead of downloading release artifacts itself.