Add support for creating a v3.0 OpenAPIKit document from an OpenAPIKit 3.1 document

[mattpolzin]

This will need to live in a new library module (OpenAPIKitCompat) that depends on both the OpenAPIKit30 and OpenAPIKit modules.

[mattpolzin]

NOTE this should be done after pretty much every other v3.x task because all other OpenAPI 3.1 changes will impact the conversion between OpenAPI versions.

[mattpolzin]

I'm not totally sure this is necessary; I think I'll leave this for after the 3.0.0 release, at least.

[mattpolzin]

It would definitely be important for any tool that read in 3.0.x and operated against the OpenAPIKit module for some logic but then wanted to write back out 3.0.x. I think I've decided this would be very nice to have.