required and default at the same time should trigger at least a warning

```json
"zoneType": {
"type": "string",
"enum": [
"Public",
"Private"
],
"x-ms-enum": {
"name": "ZoneType",
"modelAsString": false
},
"default": "Public",
"description": "The type of this DNS zone (Public or Private)."
}
},
"required": [
"zoneType"
],
```

Swagger seems to not allow this, and least Python runtime does not support it. However, Autorest does not fail, it's why I'm cautious and talk about "warning" until we have a strong agreement on it.