Allow specifying $schema on apibuilder (#2338)

src/schemas/json/apibuilder.json

@@ -618,6 +618,10 @@
   },
   "description": "JSON Schema for API Builder's api.json format from https://app.apibuilder.io/doc/apiJson",
   "properties": {
+    "$schema": {
+      "type": "string",
+      "description": "A JSON schema URL"
+    },
     "name": {
       "type": "string",
       "description": "The human readable name of this service. Used for display, and as the basis for generating a unique key for URL."

src/test/apibuilder/apibuilder-api.json

@@ -1,4 +1,5 @@
 {
+  "$schema": "../../schemas/json/apibuilder.json",
   "base_url": "https://api.apibuilder.io",
   "description": "Host API documentation for applications providing REST APIs, facilitating the design of good resource first APIs.",
   "enums": {