feat(cryptography): accept AES-GCM associated data and empty HKDF input

[matthiasgeihs]

Explanation

@metamask/cryptography can cover the Noise primitives in the MFA wallet SDK except for two gaps:

• AES-GCM encrypt and decrypt never passed associated data through to WebCrypto. Noise authenticates the handshake hash on every message, so ciphertext produced without that data does not interoperate.
• HKDF rejected a zero-length input even when unsafeInputKeyingMaterial was set. Noise Split derives the two transport keys with HKDF(chaining_key, zerolen).

additionalData is now an option on both AES-GCM calls. An empty HKDF input is accepted when unsafeInputKeyingMaterial is set; without that flag the call still throws. AES-GCM associated data is covered by NIST GCM test case 4. The empty-input HKDF vector matches Node's hkdfSync.

References

• Related to MetaMask/mfa-wallet-sdk, which uses Noise_KK with these two operations.

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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