Add working proof-of-concept of frp server (frps)

This is working on a single IPv6 address of the two we now have
avaliable. Because nginx is still binding to port 80 on all addresses,
we have to test this on port 8081, which is less than ideal. Still, it's
progress.

sovr/etc/nixos/configuration.nix

@@ -65,6 +65,15 @@
     enable = true;
     network.enable = false;
   };
+  services.frp = {
+    enable = true;
+    role = "server";
+    settings = {
+      bindAddr = "2605:e440::2:3e";
+      bindPort = 7000;
+      vhostHTTPPort = 8081;
+    };
+  };
   services.openssh = {
     enable = true;
     # require public key authentication for better security