DOC: proxy-protocol: Add SSL client certificate TLV

DOC: proxy-protocol: Add SSL client certificate TLV

Add the PP2_SUBTYPE_SSL_CLIENT_CERT code point reservation in the
proxy protocol specification. This is useful in cases where the
backend needs to perform mTLS authentication, but the rules for
certificate validation are backend-specific (e.g. database of
allowed certificate hashes).

This is left optional to leave it up to the frontend configuration
to dictate whether to forward raw certificate data.

Support for this new TLV has been added in tlstunnel:
https://codeberg.org/emersion/tlstunnel/pulls/33