Add support for arrays for the `type`

Since OAS 3.1 the `type` can take either a string value or an array / list of strings:
```
"type": "object"

"type": ["string", "integer"]
```
This has changed since OAS 3.0, where multiple types would have to be defined using `oneOf` or `anyOf`.

Currently the library does not support OAS 3.1+ specs using the array type notation.