Support validation of oneOf using OpenAPI spec Discriminator

[captain828]

Given that #99 has led to validation being disabled for oneOf and anyOf members, I suggest implementing the OpenAPI 3.0 spec discriminator.

This would only function for the oneOf use case and would be the only way to validate oneOf members. This would solve the complexity of having to validate against all models while ensuring that only one of the models is valid.

For the anyOf use case, IMO we should ignore it for now and come back to it later.

Let me know what you think about this improvement proposal. I would be happy to contribute to this if you can give me some pointers on how to set things up for dev.