Allow `anyOf` with an `enum` (to indicate "a value in this enum or a plain string") to still render the enum

i.e.

```yaml
      anyOf:
        - type: string
        - type: string
          enum:
            - rootly
            - manual
            - api
            - heartbeat
            # ...
```

Will resolve as only being a `string`.

Ideally we would keep this being a `string`, but still generate enums
