DOC: proxy-protocol: Add TLS group and sig scheme TLVs

DOC: proxy-protocol: Add TLS group and sig scheme TLVs

This change adds the PP2_SUBTYPE_SSL_GROUP and PP2_SUBTYPE_SSL_SIG_SCHEME
code point reservations in proxy_protocol.txt. The motivation for adding
these two TLVs is for backend visibility into the negotiated TLS key
exchange group and handshake signature scheme.

Demand for visibility is expected to increase as endpoints migrate to use
new Post-Quantum resistant algorithms for key exchange and signatures.