Add libopenapi-validator derived middleware

[jamietanna]

(This has been on my mind for a while, but thought it'd be good to have a tracking issue)

[PaulSonOfLars]

Hey, I'd be interested in looking into implementing this. What's the current state of your investigation? Is this something you'd be open to receiving a PR for?

If yes, would you prefer it to be a contribution to the individual middleware validation packages? Part of the codegen involved here? A built-in middleware? Or maybe even a generated middleware, similar to the spec embedding?