OpenAPI: support for union and interface types

From the example schema:
```gql
union Food = Pizza | Salad

type User {
# ...
favoriteFood: Food!
}
```

The resulting OpenAPI type for `components.schemas.User.properties.favoriteFood` is currently
```json
{
"type": "object"
}
```

Wold be nice to turn it into
```json
{
"oneOf": [
{
"$ref": "#/components/schemas/Pizza"
},
{
"$ref": "#/components/schemas/Salad"
}
]
}
```
and do the same for GraphQL interfaces.

The issue names no files or tests; begin at the OpenAPI schema generation for User.properties.favoriteFood, where the union currently becomes an object. Trace how GraphQL union and interface members are represented, then verify that both generate oneOf schemas with references to each possible type.

Written by the indexing model from the issue text.