tls: Add X25519Kyber768Draft00 PQ "curve" behind build tag (#5852)

… when compiled with cfgo (https://github.com/cloudflare/go).

modules/caddytls/cf.go

@@ -0,0 +1,24 @@
+//go:build cfgo
+
+package caddytls
+
+// This file adds support for X25519Kyber768Draft00, a post-quantum
+// key agreement that is currently being rolled out by Chrome [1]
+// and Cloudflare [2,3]. For more context, see the PR [4].
+//
+// [1] https://blog.chromium.org/2023/08/protecting-chrome-traffic-with-hybrid.html
+// [2] https://blog.cloudflare.com/post-quantum-for-all/
+// [3] https://blog.cloudflare.com/post-quantum-to-origins/
+// [4] https://github.com/caddyserver/caddy/pull/5852
+
+import (
+	"crypto/tls"
+)
+
+func init() {
+	SupportedCurves["X25519Kyber768Draft00"] = tls.X25519Kyber768Draft00
+	defaultCurves = append(
+		[]tls.CurveID{tls.X25519Kyber768Draft00},
+		defaultCurves...,
+	)
+}