[Placeholder] Proofs with duplicate HTTP headers should be treated as invalid

[themighty1]

[Note: This is not an actual issue right now (Feb 2024) since tlsn-verifier does not parse HTTP yet. But this will become an issue once we add that functionality]

Having duplicate HTTP headers in a proof may cause ambiguity for the proof verifier.
Proofs with duplicate headers should be considered invalid, except for maybe the Cookie header.

We need to have the parser expose the info about the presence of duplicate headers as discussed in tlsnotary/tlsn-utils#23