Add wrappers for crypto_aead_chacha20poly1305_ietf_encrypt(decrypt)

[beeare]

See issue in adamcaudill/libsodium-net

[tabrath]

@beeare Hi, I’ve implemented some more wrappers in a feature branch, but not yet publishet it as it need some review (I’m no crypto-expert). Might be able to create a PR soon, but cannot promise anything.

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