Add sops-nix for secret management

.sops.yaml

@@ -0,0 +1,11 @@
+# This example uses YAML anchors which allows reuse of multiple keys
+# without having to repeat yourself.
+# Also see https://github.com/Mic92/dotfiles/blob/master/nixos/.sops.yaml
+# for a more complex example.
+keys:
+  - &onizuka age1ey3wr2wnkgny3dfgvnyrf0cptwzr7s5x464p2y9ya58lpay8lfrsds3y68
+creation_rules:
+  - path_regex: secrets.yaml$
+    key_groups:
+    - age:
+      - *onizuka