Add an enumeration of client certificate modes to quic_types.h.
Add an enumeration of client certificate modes to quic_types.h.

ClientCertMode is used to configure server endpoints.  It tells the endpoint
whether it should request a client certificate, and whether it should allow
"unauthenticated" connections (when the client does not have a certificate).

TLS 1.3 supports three modes for the server:
 - No client certificate requested
 - Request a client certificate but allow unauthenticated connections
 - Request a client certificate and reject unauthenticated connections

See go/quic-tls-client-certificates for details.

gfe-relnote: no change in behavior (new enum definition only).
PiperOrigin-RevId: 291452671
Change-Id: Iaab64381451af7ea0cd2a324d3262ae1e46d0bcd

google/quiche,google/quiche,google/quiche,google/quiche