Add support for mapping newer OAS versions to older ones

[mattpolzin]

New Feature

OpenAPIKit adds support for new OAS versions when it has support for most or
all of the features of that OAS version. If you want to parse an OpenAPI
Document that is written in a newer version than OpenAPIKit supports and you
are asserting that the newer version is possible to parse as if it were the
pre-existing version, you can tell OpenAPIKit to parse the newer version as if
it were the older version.

You do this with userInfo passed into the Decoder you are using. For
example, to decode a hypothetical document version of "3.100.100" as if it
were version "3.1.1", set your decoder up as follows:

let userInfo = [
  DocumentConfiguration.versionMapKey: ["3.100.100": OpenAPI.Document.Version.v3_1_1]
]

let decoder = ... // JSONDecoder() or YAMLDecoder()
decoder.userInfo = userInfo

let openAPIDoc = try decoder.decode(OpenAPI.Document.self, from: ...)