feat: OpenAPI v3.0 support

[Marsup]

Hi,

I've implemented basic OpenAPI v3.0 spec output with the additional support for anyOf and nullable in JSON schemas.

I started without knowing Swagger 2.0, OpenAPI 3.0, or this module at all, so there may be mistakes, but it seems to pass validators. I'm pretty sure I didn't implement the full spec, but it seems to work on all the test servers and on our own real-world server, so I think it's already a good starting point.

I tried to reuse most of the existing code whenever possible, but working with the code base seemed a bit tedious at the end. I feel like this may need a deeper rewrite, but I didn't have the resources nor did I feel entitled to do that in a feature PR, so I hope the way I did it won't bother you too much. Tests are sometimes adapted, sometimes full copies of existing ones with the 3.0 twist when it would introduce too many conditionals, it feels more maintainable that way, but one has to implement new tests in both files in the future, unfortunately. This may be possible with a lot of abstracted test helpers.

[robmcguinness]

@Marsup Thank you for PR. I will be reviewing shortly. I agree with all your points.

[Marsup]

Thanks a lot Rob. For the record, I'm still ironing out some quirks I find now that I'm integrating it into other tools, but it's stable enough.