Support for propertyNames key/value-objects etc

I am trying to set up a form from an object that is of type `Record`. I'm not sure if this is already supported.

I've tried this schema (showing only the relevant part):

```json
{
"clients": {
"additionalProperties": {
"additionalItems": true,
"description": "One or more valid password that can be used for client-authentication",
"items": {
"type": "string"
},
"minItems": 1,
"title": "The clientId schema",
"type": "array",
"uniqueItems": true
},
"description": "The clients will authenticate via Basic Auth",
"examples": [
{
"clientId": [
"password1",
"password2"
]
}
],
"propertyNames": {
"maxLength": 100,
"minLength": 3
},
"title": "As an alternative to other authentication, a simple map of client with clientSecrets can be supplied",
"type": "object"
}
}
```

Perhaps this only needs a custom renderer? I'm thinking it should act sort of like the Material-renderer does already with arrays, only that one has to provide the propertyName as well.