safeweb: add Server.Close method (#13160)

Updates https://github.com/tailscale/corp/issues/14881

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

safeweb/http.go

@@ -300,3 +300,8 @@ func (s *Server) ServeRedirectHTTP(ln net.Listener, fqdn string) error {
 func (s *Server) Serve(ln net.Listener) error {
 	return s.h.Serve(ln)
 }
+
+// Close closes all client connections and stops accepting new ones.
+func (s *Server) Close() error {
+	return s.h.Close()
+}