Try to add nix-darwin support to agenix

Merges work by @montchr, @cmhamill, and @rtimush and rebases on main.

- fixes https://github.com/ryantm/agenix/issues/60
- fixes https://github.com/ryantm/agenix/issues/120
- closes https://github.com/ryantm/agenix/pull/107

test/install_ssh_host_keys_darwin.nix

@@ -0,0 +1,10 @@
+# Do not copy this! It is insecure. This is only okay because we are testing.
+{
+  system.activationScripts.extraUserActivation.text = ''
+    echo "Installing SSH host key"
+    sudo cp ${../example_keys/system1.pub} /etc/ssh/ssh_host_ed25519_key.pub
+    sudo cp ${../example_keys/system1} /etc/ssh/ssh_host_ed25519_key
+    sudo chmod 644 /etc/ssh/ssh_host_ed25519_key.pub
+    sudo chmod 600 /etc/ssh/ssh_host_ed25519_key
+  '';
+}

test/integration_darwin.nix

@@ -0,0 +1,24 @@
+{
+  config,
+  pkgs,
+  ...
+}: let
+  secret = "hello";
+  testScript = pkgs.writeShellApplication {
+    name = "agenix-integration";
+    text = ''
+      grep ${secret} ${config.age.secrets.secret1.path}
+    '';
+  };
+in {
+  imports = [
+    ./install_ssh_host_keys_darwin.nix
+    ../modules/age.nix
+  ];
+
+  services.nix-daemon.enable = true;
+
+  age.secrets.secret1.file = ../example/secret1.age;
+
+  environment.systemPackages = [testScript];
+}