Refactor imports and secrets. Add per system properties and role based secret access.

Highlights
- No need to update flake for every machine anymore, just add a properties.nix file.
- Roles are automatically generated from all machine configurations.
- Roles and their secrets automatically are grouped and show up in agenix secrets.nix
- Machines and their service configs may now query the properties of all machines.
- Machine configuration and secrets are now competely isolated into each machine's directory.
- Safety checks to ensure no mixing of luks unlocking secrets and hosts with primary ones.
- SSH pubkeys no longer centrally stored but instead per machine where the private key lies for better cleanup.

common/machine-info/roles.nix

@@ -0,0 +1,19 @@
+{ config, lib, ... }:
+
+# Maps roles to their hosts
+
+{
+  options.machines.roles = lib.mkOption {
+    type = lib.types.attrsOf (lib.types.listOf lib.types.str);
+  };
+
+  config = {
+    machines.roles = lib.zipAttrs
+      (lib.mapAttrsToList
+        (host: cfg:
+          lib.foldl (lib.mergeAttrs) { }
+            (builtins.map (role: { ${role} = host; })
+              cfg.systemRoles))
+        config.machines.hosts);
+  };
+}