Implement AES-256-GCM encryption

[dmjb]

Fixes #3317

The GCM mode is recommended by OWASP and other authorities for secret encryption. As per @jhrozek's suggestion, use the cryptopasta implementation of AES-256-GCM (see link/copyright in code).

This PR does not actually make use of the new algorithm yet, this will be done in a future PR.

Summary

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Fixes #(related issue)

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• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
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Review Checklist:

• Reviewed my own code for quality and clarity.
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• Updated any affected documentation.
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[coveralls]

coverage: 50.394% (+0.06%) from 50.336%
when pulling 59bc23c on new-crypto-algorithm
into 1ccdb36 on main.