Add code signing to build workflow

[jmbowman]

Add an option to sign the macOS and Windows installers for manual runs of the GitHub Actions build workflow. Don't allow workflows run automatically for commits to sign their installers.

Some articles that may help in setting this up:

• https://federicoterzi.com/blog/automatic-code-signing-and-notarization-for-macos-apps-using-github-actions/
• https://localazy.com/blog/how-to-automatically-sign-macos-apps-using-github-actions
• https://docs.github.com/en/actions/security-for-github-actions/security-guides/using-secrets-in-github-actions