crypto/tls: add Extensions to ClientHelloInfo #32936
Description
Similar aws/s2n-tls#607, Having access to raw ClientHello can be useful for fingerprinting clients [1] for further analysis.
Plus, With raw ClientHello message, we could also implements SNI Proxy in tls.Config.GetConfigForClient [2] , e.g. tlsrouter [3] more easily.
In openssl this can be done by setting up callback through SSL_CTX_set_msg_callback.
Would be nice to have similar ability for golang crypto.
[1] https://github.com/salesforce/ja3
[2] https://golang.org/pkg/crypto/tls/#Config
[3] https://github.com/google/tcpproxy/tree/master/cmd/tlsrouter