Add current IP automatically to whitelists for SSH and Kubernetes

After I removed the automatic IP addition to the firewalls for SSH and
Kubernetes I ran into a problem only a few days later. My ISP changed
my IPs and I was to stupid to realize that immediately. So, this change
reintroduces the automatic addition of my current IPs to the whitelists
for Kubernetes and SSH. Though, I adjusted the algorithm, so it will not
change every day or so, but instead really only when my ISP changes my
IPs.

versions.tf

@@ -26,6 +26,10 @@ terraform {
       source  = "hashicorp/random"
       version = "3.7.1"
     }
+    external = {
+      source  = "hashicorp/external"
+      version = "2.3.4"
+    }
   }
 }
 
@@ -38,3 +42,5 @@ provider "hcloud" {
 }
 
 provider "random" {}
+
+provider "external" {}