openapi2,3: support array of types in type field

[brandonbloom]

Resolves #563

The discussion in issue #563 mentions that this is only supported by openapi2, but the JSON Schema validation spec seems to imply that this should still be supported: https://json-schema.org/draft/2020-12/json-schema-validation#name-type

If the value of "type" is an array, then an instance validates successfully if its type matches any of the types indicated by the strings in the array.

This PR is a big and breaking change and only really adds two tests. Putting it up for discussion sake. Totally understood if an alternative approach is preferred.

[fenollp]

This approach is the way forward to me too. Thanks for tackling this!

Just a comment, really yack shaving on a test...
More importantly, please push the diff generated by that command:

./docs.sh

[fenollp]

Let's update this test so its output is serialized json (or yaml), so as to show that serialization turns single valued arrays of types into a string

[brandonbloom]

I think that's precisely what this comment change is doing, right? Not sure I follow the concern.

[fenollp]

The discussion in issue #563 mentions that this is only supported by openapi2, but the JSON Schema validation spec seems to imply that this should still be supported

Note that OpenAPIvN when N<3.1 does not use the JSON Schema draft, it defines its own JSON schema.
So in openapi3's schema.go there should be a new Validate() check that returns an error IFF len(schema.Type) >1.
And in the v2-to-v3 code here's the comment: #563 (comment)

[brandonbloom]

I've run ./docs.sh and resolved conflicts with mainline changes.

Regarding the legality of types being an array, it's actually valid again in 3.1. See discussion here: mattpolzin/OpenAPIKit#356

If you plan on merging this change, I'd appreciate it you did it sooner rather than later and then we followed up with any smaller remaining improvements. Due to the pervasive nature of this change, conflicts are very likely with other changes, as evidenced by the conflicts I had to resolve before I could update this branch. Thanks!

[brandonbloom]

(accidentally closed, reopening now)

[brandonbloom]

Thanks!