crypto/tls: support QUIC as a transport

Add a QUICConn type for use by QUIC implementations.

A QUICConn provides unencrypted handshake bytes and connection
secrets to the QUIC layer, and receives handshake bytes.

For #44886

Change-Id: I859dda4cc6d466a1df2fb863a69d3a2a069110d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/493655
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Marten Seemann <martenseemann@gmail.com>

alert.go

@@ -6,6 +6,16 @@ package tls
 
 import "strconv"
 
+// An AlertError is a TLS alert.
+//
+// When using a QUIC transport, QUICConn methods will return an error
+// which wraps AlertError rather than sending a TLS alert.
+type AlertError uint8
+
+func (e AlertError) Error() string {
+	return alert(e).String()
+}
+
 type alert uint8
 
 const (