Add sample for opkssh module

Module will not be added here as usernames, principals and the client id
have to be specified directly.
Setting them via age secrets is not possible.

modules/opkssh.sample.nix

@@ -0,0 +1,18 @@
+{
+  ...
+}: {
+  services.opkssh = {
+    enable = true;
+    providers = {
+      pocket-id = {
+        issuer = "https://example.com";
+        clientId = "";
+        lifetime = "12h";
+      };
+    };
+    authorizations = [
+      { user = "<username>"; principal = "<email>"; issuer = "https://example.com"; }
+    ];
+  };
+}
+