api/machines: allow importing extra modules

- add top-level option `clanImports` to clanCore
- clanImports can be set and checked as any other option
- buildClan resolves the clanImports from the settings.json before calling evalModules to prevent infinite recursions
- new endpoint PUT machines/{name}/schema to allow getting the schema for a specific list of imports
- to retrieve the currently imported modules, cimply do a GET or PU on machines/{name}/config which will return `clanImports` as part of the config

Still missing: get list of available modules

nixosModules/clanImports/default.nix

@@ -0,0 +1,16 @@
+{ lib
+, ...
+}: {
+  /*
+    Declaring imports inside the module system does not trigger an infinite
+    recursion in this case because buildClan generates the imports from the
+    settings.json file before calling out to evalModules.
+  */
+  options.clanImports = lib.mkOption {
+    type = lib.types.listOf lib.types.str;
+    description = ''
+      A list of imported module names imported from clan-core.clanModules.<name>
+      The buildClan function will automatically import these modules for the current machine.
+    '';
+  };
+}