fix: fix type of payload in MessageExample (#561)

* fix: fix type of payload and headers in MessageExample

#560

* fix: wrong type of payload and headers in MessageExample

Rollback headers to Map[string, any]

* fix: headers description in MessageExample

Update headers description. Clarify that headers must be a map of key-value pairs

* Update definitions/3.0.0/messageExampleObject.json

Co-authored-by: Fran Méndez <fmvilas@gmail.com>

---------

Co-authored-by: Fran Méndez <fmvilas@gmail.com>

Author: Pakisan

definitions/3.0.0/messageExampleObject.json

@@ -24,9 +24,10 @@
     },
     "headers": {
       "type": "object",
-      "description": "Example of the application headers. It can be of any type."
+      "description": "Example of the application headers. It MUST be a map of key-value pairs."
     },
     "payload": {
+      "type": ["number", "string", "boolean", "object", "array", "null"],
       "description": "Example of the message payload. It can be of any type."
     }
   },