crypto/tls: add VerifyPeerCertificate to tls.Config

[jboelter]

It would be useful to have a callback in tls.Config called after the TLS handshake to allow the callee to perform additional or alternate certificate verification on the peer. This would enable custom cert checking and/or explicit certificate pinning/whitelisting (e.g. by cert hash).

I propose calling VerifyPeerCertificate (if !nil) during the respective client and server handshakes (doFullHandshake and processPeerCertsFromClient) after the peer certificates have been received.

The func is introduced to tls.Config

    // VerifyPeerCertificate returns an error if the peer should not be
    // trusted. It will be called after the initial handshake and before
    // any other verification checks on the cert or chain are performed.
    // This provides the callee an opportunity to augment the certificate
    // verification.
    //
    // If VerifyPeerCertificate is not nil and returns an error,
    // then the handshake will fail.
    VerifyPeerCertificate func(peer []*x509.Certificate) error

This works with both http and http2. It appears that tls.Config is cloned from the initial transport for TLSNextProto -- is this intentional or an artifact of the current impl?

    c := &http.Client{
        Transport: &http.Transport{
            Proxy: http.ProxyFromEnvironment,
            Dial: (&net.Dialer{
                Timeout:   60 * time.Second,
                KeepAlive: 60 * time.Second,
            }).Dial,
            TLSHandshakeTimeout:   10 * time.Second,
            ExpectContinueTimeout: 1 * time.Second,
            TLSClientConfig: &tls.Config{
                // inject our callback to check the peer here
                NextProtos:            []string{"h2"},
                VerifyPeerCertificate: verifyPeerCertificate,
            },
            TLSNextProto: map[string]func(authority string, c *tls.Conn) http.RoundTripper{
                "h2": func(authority string, c *tls.Conn) http.RoundTripper {
                    return http.DefaultTransport
                },
            },
        },
    }

I have a changset ready if this is a reasonable approach.

Fixes #9126
Fixes #9451

[ianlancetaylor]

CC @agl

[gopherbot]

CL https://golang.org/cl/26654 mentions this issue.