Create a `format` Registry

[webron]

In reference to #607 and #811.

Instead of adding additional formats to the spec, we're considering two options - either a set of formats in a supporting guidelines document or a formal OAI format registry with a set of guidelines how to enter new formats to it. The registry will serve as an official repository for these formats and tools could use it as a reference. The @OAI/tdc is currently leaning towards the option of the repository.

This ticket is a reminder to tackle it and finalize the approach and the guidelines for either case.

[MikeRalphson]

Summary of existing formats used in the wild. https://github.com/Mermade/openapi-specification-extensions/blob/master/formats/combined.tsv

It may be worth recommending that format strings only contain lower-case letters, digits and hyphens, or some other restriction, to draw attention to the fact it is not intended as an essay on the content of the property or an enum, pattern or template.