engine: Allow $schema key to be present in a cloe stack file

Author: cassava

engine/src/stack.hpp

@@ -880,6 +880,7 @@ using ConfReader = std::function<Conf(const std::string&)>;
 class Stack : public Confable {
  private:  // Constants (1)
   std::vector<std::string> reserved_ids_;
+  boost::optional<std::string> schema_ref_;
 
  public:  // Configuration (13)
   EngineConf engine;
@@ -1085,6 +1086,7 @@ class Stack : public Confable {
     using namespace schema;  // NOLINT(build/namespaces)
 
     return Struct{
+        {"$schema", make_schema(&schema_ref_, "valid URI to schema describing this cloe stack version")},
         {"version", make_const_str(CLOE_STACK_VERSION, "version of stackfile").require()},
         {"engine", engine_schema},
         {"include", include_schema},

engine/tests/test_engine_json_schema.json

@@ -1,6 +1,17 @@
 {
   "additionalProperties": false,
   "properties": {
+    "$schema": {
+      "description": "valid URI to schema describing this cloe stack version",
+      "oneOf": [
+        {
+          "type": "null"
+        },
+        {
+          "type": "string"
+        }
+      ]
+    },
     "controllers": {
       "description": "controller configuration",
       "items": {