Use AAD as input of Chacha20Poly1305.encrypt()

[cygnusv]

We're currently relying on validating the AAD as part of the TPKE public verifiability function, but not as part of Chacha20Poly1305's properties as an AEAD cipher. That is, as a defense in depth measure, we could include the AAD as input when encrypting & decrypting with Chacha20Poly1305.

See https://docs.rs/aead/0.5.1/aead/struct.Payload.html