derp/xdp,cmd/xdpderper: initial skeleton (#12390)

This commit introduces a userspace program for managing an experimental
eBPF XDP STUN server program. derp/xdp contains the eBPF pseudo-C along
with a Go pkg for loading it and exporting its metrics.
cmd/xdpderper is a package main user of derp/xdp.

Updates tailscale/corp#20689

Signed-off-by: Jordan Whited <jordan@tailscale.com>

derp/xdp/xdp_default.go

@@ -0,0 +1,28 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux
+
+package xdp
+
+import (
+	"errors"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// STUNServer is unimplemented on these platforms, see xdp_linux.go.
+type STUNServer struct {
+}
+
+func NewSTUNServer(config *STUNServerConfig, opts ...STUNServerOption) (*STUNServer, error) {
+	return nil, errors.New("unimplemented on this GOOS")
+}
+
+func (s *STUNServer) Close() error {
+	return errors.New("unimplemented on this GOOS")
+}
+
+func (s *STUNServer) Describe(descCh chan<- *prometheus.Desc) {}
+
+func (s *STUNServer) Collect(metricCh chan<- prometheus.Metric) {}