Only sender pubkey in hpke.base metadata seal

[rocodes]

Refs #86 (comment)

Description

Changes to benchmarking code:

• Moved the DH-AKEM encaps shared secret and the PQ PSK encaps shared secret out of the hpke.base encrypted "metadata" and directly into the Envelope (the message payload to the server) per followup discussion with ethz - no need to double-encrypt them. (This matches the manuscript).
  • That means there's no "metadata" type now
  • It theoretically means that there should be a wrapper type for the ciphertext in the Envelope (to keep with the C, X, Z format), but since it's a quick benchmark implementation I have left all the bytes as separate types rather than concatenating them and them parsing them. edit: there's a wrapper type. :)
  • (There's the CombinedCiphertext type for serializing/deserializing all the parts, but I kept the type signature and even variable name in the Envelope struct unchanged, to avoid having to make any wasm-bindgen changes).
• bonus: Include keygen for journalist reply keys (they aren't used because we aren't benching replies yet, but for more completeness).

Test plan

• Visual review
• CI
• Compare with manuscript and discussion
• make bench

[rocodes]

(rebased, force-pushed, base branch now main)

[cfm]

Thanks, @rocodes.