Attestation type negotiation via ALPN

[ameba23]

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.

[ameba23]

This has been mostly addressed with #36 but leaving it open as i would consider also doing attestation type negotiation via ALPN. That is, agree the attestation platforms used by server and client during the initial TLS handshake.