allow DB specific mappings for indexes

Consistent with mysql connector, passing a "kind" attribute to an index will allow to create 2dsphere or text indexes.
allow DB specific mappings for indexes

```
"geometry": {
  "type": "Object",
  "required": false,
  "index": {
    "mongodb": {
      "kind": "2dsphere"
    }
  }
}
```

fixes detection of root level property 'unique'