feat: allow reusability of Server Variable Object (#250)

Author: char0n

definitions/2.5.0/serverVariables.json

@@ -1,7 +1,14 @@
 {
   "type": "object",
   "additionalProperties": {
-    "$ref": "http://asyncapi.com/definitions/2.5.0/serverVariable.json"
+    "oneOf": [
+      {
+        "$ref": "http://asyncapi.com/definitions/2.5.0/Reference.json"
+      },
+      {
+        "$ref": "http://asyncapi.com/definitions/2.5.0/serverVariable.json"
+      }
+    ]
   },
   "$schema": "http://json-schema.org/draft-07/schema#",
   "$id": "http://asyncapi.com/definitions/2.5.0/serverVariables.json"