crypto/tls: implement TLS 1.3 version-specific messages

Note that there is significant code duplication due to extensions with
the same format appearing in different messages in TLS 1.3. This will be
cleaned up in a future refactor once CL 145317 is merged.

Enforcing the presence/absence of each extension in each message is left
to the upper layer, based on both protocol version and extensions
advertised in CH and CR. Duplicated extensions and unknown extensions in
SH, EE, HRR, and CT will be tightened up in a future CL.

The TLS 1.2 CertificateStatus message was restricted to accepting only
type OCSP as any other type (none of which are specified so far) would
have to be negotiated.

Updates #9671

Change-Id: I7c42394c5cc0af01faa84b9b9f25fdc6e7cfbb9e
Reviewed-on: https://go-review.googlesource.com/c/145477
Reviewed-by: Adam Langley <agl@golang.org>

common.go

@@ -56,19 +56,23 @@ const (
 
 // TLS handshake message types.
 const (
-	typeHelloRequest       uint8 = 0
-	typeClientHello        uint8 = 1
-	typeServerHello        uint8 = 2
-	typeNewSessionTicket   uint8 = 4
-	typeCertificate        uint8 = 11
-	typeServerKeyExchange  uint8 = 12
-	typeCertificateRequest uint8 = 13
-	typeServerHelloDone    uint8 = 14
-	typeCertificateVerify  uint8 = 15
-	typeClientKeyExchange  uint8 = 16
-	typeFinished           uint8 = 20
-	typeCertificateStatus  uint8 = 22
-	typeNextProtocol       uint8 = 67 // Not IANA assigned
+	typeHelloRequest        uint8 = 0
+	typeClientHello         uint8 = 1
+	typeServerHello         uint8 = 2
+	typeNewSessionTicket    uint8 = 4
+	typeEndOfEarlyData      uint8 = 5
+	typeEncryptedExtensions uint8 = 8
+	typeCertificate         uint8 = 11
+	typeServerKeyExchange   uint8 = 12
+	typeCertificateRequest  uint8 = 13
+	typeServerHelloDone     uint8 = 14
+	typeCertificateVerify   uint8 = 15
+	typeClientKeyExchange   uint8 = 16
+	typeFinished            uint8 = 20
+	typeCertificateStatus   uint8 = 22
+	typeKeyUpdate           uint8 = 24
+	typeNextProtocol        uint8 = 67  // Not IANA assigned
+	typeMessageHash         uint8 = 254 // synthetic message
 )
 
 // TLS compression types.
@@ -87,6 +91,7 @@ const (
 	extensionSCT                     uint16 = 18
 	extensionSessionTicket           uint16 = 35
 	extensionPreSharedKey            uint16 = 41
+	extensionEarlyData               uint16 = 42
 	extensionSupportedVersions       uint16 = 43
 	extensionCookie                  uint16 = 44
 	extensionPSKModes                uint16 = 45