Add a ServerProofVerifier and ClientCertMode to QuicCryptoServerConfig. These two parameters allow configuration of server endpoints that request (or require) client certificates. This cl adds the settings, but implementation is left to a future change. See go/quic-tls-client-certificates for details. gfe-relnote: no behavior change (new settings are not used in gfe) PiperOrigin-RevId: 291452792 Change-Id: Ia2a6299d51bec8446043ac859087882c5cac3b88