crypto/tls: expose extensions presented by client to GetCertificate

This enables JA3 and JA4 TLS fingerprinting to be implemented from
the GetCertificate callback, similar to what BoringSSL provides with
its SSL_CTX_set_dos_protection_cb hook.

fixes #32936

Change-Id: Idb54ebcb43075582fcef0ac6438727f494543424
Reviewed-on: https://go-review.googlesource.com/c/go/+/471396
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

common.go

@@ -446,6 +446,10 @@ type ClientHelloInfo struct {
 	// might be rejected if used.
 	SupportedVersions []uint16
 
+	// Extensions lists the IDs of the extensions presented by the client
+	// in the client hello.
+	Extensions []uint16
+
 	// Conn is the underlying net.Conn for the connection. Do not read
 	// from, or write to, this connection; that will cause the TLS
 	// connection to fail.

handshake_messages.go

@@ -97,6 +97,8 @@ type clientHelloMsg struct {
 	pskBinders                       [][]byte
 	quicTransportParameters          []byte
 	encryptedClientHello             []byte
+	// extensions are only populated on the server-side of a handshake
+	extensions []uint16
 }
 
 func (m *clientHelloMsg) marshalMsg(echInner bool) ([]byte, error) {
@@ -467,6 +469,7 @@ func (m *clientHelloMsg) unmarshal(data []byte) bool {
 			return false
 		}
 		seenExts[extension] = true
+		m.extensions = append(m.extensions, extension)
 
 		switch extension {
 		case extensionServerName:

handshake_server.go

@@ -949,6 +949,7 @@ func clientHelloInfo(ctx context.Context, c *Conn, clientHello *clientHelloMsg)
 		SignatureSchemes:  clientHello.supportedSignatureAlgorithms,
 		SupportedProtos:   clientHello.alpnProtocols,
 		SupportedVersions: supportedVersions,
+		Extensions:        clientHello.extensions,
 		Conn:              c.conn,
 		config:            c.config,
 		ctx:               ctx,