Attestation type negotiation via ALPN

ALPN protocol negotiation (part of TLS) lets us specify supported protocols during the handshake.

We can use this to negotiate different accepted attestation types before the session begins.

Start by tracing the TLS handshake and the existing attestation-type handling on both proxy sides. Determine how ALPN values should represent supported attestation types and how negotiation affects the session. Done means the handshake negotiates an accepted type before the session begins, with behavior defined for unsupported or incompatible types.

Written by the indexing model from the issue text.