pyrocufflink-dns: Cloudflare over ProtonVPN

This commit adds a new playbook, `protonvpn.yml`, and its supporting
roles *strongswan-swanctl* and *protonvpn*.  This playbook configures
strongSwan to connect to ProtonVPN using IPsec/IKEv2.

With this playbook, we configure the name servers on the Pyrocufflink
network to route all DNS requests through the Cloudflare public DNS
recursive servers at 1.1.1.1/1.0.0.1 over ProtonVPN.  Using this setup,
we have the benefit of the speed of using a public DNS server (which is
*significantly* faster than running our own recursive server, usually by
1-2 seconds per request), and the benefit of anonymity from ProtonVPN.

Using the public DNS server alone is great for performance, but allows
the server operator (in this case Cloudflare) to track and analyze usage
patterns.  Using ProtonVPN gives us anonymity (assuming we trust
ProtonVPN not to do the very same tracking), but can have a negative
performance impact if its used for all Internet traffic.  By combining
these solutions, we can get the benefits of both!

roles/strongswan-swanctl/tasks/main.yml

@@ -0,0 +1,15 @@
+- name: ensure strongswan is installed
+  package:
+    name: strongswan
+    state: present
+  tags:
+  - install
+
+- name: ensure strongswan starts at boot
+  service:
+    name: strongswan
+    enabled: true
+- name: ensure strongswan is running
+  service:
+    name: strongswan
+    state: started