Support of "oneOf, anyOf, allOf"

I'd like to have ability to configure my model to having something like this output:
```
"fieldName": {
"type": "array",
"items": {
"oneOf": [
{ "$ref": "#/definitions/item" },
{ "$ref": "#/definitions/itemGroup" }
]
}
}
```

The issue names no files, tests, or entry points and provides only a target OpenAPI schema example. Start by tracing how model schemas are generated and how configuration is exposed; done means the requested oneOf, anyOf, and allOf output can be produced for the described array case.

Written by the indexing model from the issue text.