Support decode of multiple JSON Schema types (other than null)

[mattpolzin]

OpenAPIKit supports combining the JSON Schema null type with one other type as of #210. This allows us to continue to represent schema nullability in an OpenAPI v3.1 document.

What is missing is the ability to decode other combinations of arbitrarily many JSON Schema types. The JSON Schema spec allows it, so OpenAPI v3.1 allows, so we should be able to decode it. I think one way to support this without making drastic changes to the OpenAPIKit JSONSchema types would be to decode "type": ["string", "number"] as if it were a oneOf -- my hope is we do not lose any expressiveness this way. That will mean that encoding a document having decoded it will result in different structure, but if it is at least comparable in what documentation it represents then I think this is an acceptable area for OpenAPIKit to be opinionated about the encoding of the representation.