xcode: allow ICMP ping relay on macOS + iOS platforms (#12048)

Fixes tailscale/tailscale#10393
Fixes tailscale/corp#15412
Fixes tailscale/corp#19808

On Apple platforms, exit nodes and subnet routers have been unable to relay pings from Tailscale devices to non-Tailscale devices due to sandbox restrictions imposed on our network extensions by Apple. The sandbox prevented the code in netstack.go from spawning the `ping` process which we were using.

Replace that exec call with logic to send an ICMP echo request directly, which appears to work in userspace, and not trigger a sandbox violation in the syslog.

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

wgengine/netstack/netstack_userping_apple.go

@@ -0,0 +1,38 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build darwin || ios
+
+package netstack
+
+import (
+	"net/netip"
+	"time"
+
+	"github.com/prometheus-community/pro-bing"
+)
+
+// sendOutboundUserPing sends a non-privileged ICMP (or ICMPv6) ping to dstIP with the given timeout.
+func (ns *Impl) sendOutboundUserPing(dstIP netip.Addr, timeout time.Duration) error {
+	p, err := probing.NewPinger(dstIP.String())
+	if err != nil {
+		ns.logf("sendICMPPingToIP failed to create pinger: %v", err)
+		return err
+	}
+
+	p.Timeout = timeout
+	p.Count = 1
+	p.SetPrivileged(false)
+
+	p.OnSend = func(pkt *probing.Packet) {
+		ns.logf("sendICMPPingToIP: forwarding ping to %s:", p.Addr())
+	}
+	p.OnRecv = func(pkt *probing.Packet) {
+		ns.logf("sendICMPPingToIP: %d bytes pong from %s: icmp_seq=%d time=%v", pkt.Nbytes, pkt.IPAddr, pkt.Seq, pkt.Rtt)
+	}
+	p.OnFinish = func(stats *probing.Statistics) {
+		ns.logf("sendICMPPingToIP: done, %d replies received", stats.PacketsRecv)
+	}
+
+	return p.Run()
+}