http3: add http3 package for testing in std

This CL adds an initial http3 package that serves as a thin wrapper over
the exported functions in the internal/http3 package. Doing so allows us
to import and vendor the HTTP/3 implementation in x/net from std. This
will be used to run std tests against our HTTP/3 implementation.

To prevent users from unexpectedly using this package, only unexported
symbols have been added to the package, with linkname that only allows
them to be used by the net/http_test package in std.

For golang/go#70914

Change-Id: I6b420bb1eb07fd86174ee4c1ede43ef26a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/762041
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Auto-Submit: Nicholas Husin <husin@google.com>

http3/http3.go

diff --git a/http3/http3.go b/http3/http3.go
new file mode 100644
index 0000000..72ac090
--- /dev/null
+++ b/http3/http3.go
@@ -0,0 +1,31 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package http3
+
+import (
+	"net/http"
+	_ "unsafe" // for linkname
+
+	. "golang.org/x/net/internal/http3"
+	"golang.org/x/net/quic"
+)
+
+//go:linkname registerHTTP3Server net/http_test.registerHTTP3Server
+func registerHTTP3Server(s *http.Server) <-chan string {
+	listenAddr := make(chan string)
+	RegisterServer(s, ServerOpts{
+		ListenQUIC: func(addr string, config *quic.Config) (*quic.Endpoint, error) {
+			e, err := quic.Listen("udp", addr, config)
+			listenAddr <- e.LocalAddr().String()
+			return e, err
+		},
+	})
+	return listenAddr
+}
+
+//go:linkname registerHTTP3Transport net/http_test.registerHTTP3Transport
+func registerHTTP3Transport(tr *http.Transport) {
+	RegisterTransport(tr)
+}