Add macOS and Windows code signing to release workflow

[tomerlir]

What

Adds code signing steps to the release pipeline for macOS and Windows binaries. macOS binaries are signed with a Developer ID certificate and notarized via Apple's notarization service. Windows binaries are signed with a PFX certificate via signtool. Both steps are skipped gracefully if secrets aren't configured.

Why

Without signing, macOS Gatekeeper and Windows SmartScreen block or warn users when running Bridge binaries. Closes internal ticket.

Checklist

• I have signed the CLA (first-time contributors)
• Tests added or updated
• cargo test passes
• cargo fmt --check passes
• cargo clippy passes

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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