Allow untyped objects to be specified as bytes

[Dan Maas]

In many of our specs, we have things like this:

      authorization_data:
        type: object
        description: Data, as an arbitrary json blob

or

      authorization_data:
        type: object
        description: Data, as an arbitrary json blob
        additionalProperties: true

This allows arbitrary data, per https://swagger.io/docs/specification/data-models/dictionaries/

Currently, this will be generated as an empty Message or a map<string, empty Message> by this library. This does't work well. My proposal is that these should actually be modeled as the scalar type bytes. That way, whatever language implementation is using the schema can decide how to convert the arbitrary data into and from bytes.

Another option would be to use google.protobuf.Any, as it does basically the same thing, but also allows packing/unpacking to different Message types.

@jprobinson any thoughts on what is preferred?

[Dan Maas]

Looking into the further, it seems like using Any is the most flexible option.

[Dan Maas]

I take that back - Any is intended to represent any other Message type. It seems a poor fit. I'm not sure what the best solve is for this.

[Dan Maas]

I updated my PR to use Struct. Let me know your thoughts.