use single json schema for key and value

[shortishly]

An example JSON schema for the "person" topic:

{
  "title": "Person",
  "type": "object",
  "properties": {

    "key": {
      "type": "string",
      "pattern": "^[A-Z]{3}-\\d{3}$"
    },

    "value": {
      "type": "object",
      "properties": {
        "firstName": {
          "type": "string",
          "description": "The person's first name."
        },
        "lastName": {
          "type": "string",
          "description": "The person's last name."
        },
        "age": {
          "description": "Age in years which must be equal to or greater than zero.",
          "type": "integer",
          "minimum": 0
        }
      }
    }
  }
}

The schema must be an object, with properties for the message "key" and/or "value".