refactor!: replace ZeroTier with Tailscale and Wireguard

- Tailscale UX is better, and the Headscale control server is also
  easier to self-host than ZeroTier (although Headscale is not the
  official control server, the author now works at Tailscale)
- Wireguard is also added as an alternative to avoid relying on a
  third-party service, however it requires port-forwarding

scripts/get-wireguard-config

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+set -eu
+
+PEER="${1}"
+
+export KUBECONFIG=./metal/kubeconfig.yaml
+
+kubectl -n wireguard exec -it deployment/wireguard -- /app/show-peer "${PEER}"
+kubectl -n wireguard exec -it deployment/wireguard -- cat "/config/peer_${PEER}/peer_${PEER}.conf"