Support for base64 encoded files

OpenAPI v3 supports following formats of binary data:
```yaml
type: string
format: binary # binary file contents
```
and
```yaml
type: string
format: byte # base64-encoded file contents
```

Swagger 2 supports:
```yaml
type: file
```

Swagger 2 `type:file` is internally mapped to `format:binary` mentioned above.

We do have a support for `format: binary` but we lack support for `format: byte`. We should add it.

The easy approach is to generate type of `java.io.File` for both formats but some information is lost if we do it that way and we force users to their own base64 encoding/decoding. If we changed to type generated for files from `java.io.File` to perhaps `{Input/Output}Stream` or even `fs2.Stream`, it would perhaps allow us to do the conversion for them in the right places.

Thoughts?