refactor(Keys): Check with crypto research if we can use poseidon for aes secret

[LHerskind]

Currently the keys spec uses different methods of deriving symmetric keys for AES using a few different methods. Mainly the incoming body secrets and headers are using SHA while the outgoing is using poseidon.

Ideally we could get away with one flow as it makes the code a bit simpler to figure out, but we need to validate this with the crypto team.