Supporting newer drafts

[DannyvdSluijs]

I guess we are running behind on supporting the newest/newer drafts. And now seems to be the time. As I've replied in #395 what could help at this point in time is:

• Proof of Concept PR: Draft a PR that adds context of which draft version is being validated
• Research: How would validating work if we reference an schema that uses a different version draft, should we switch the draft version (and switch back when leaving the schema) or keep the current schema. I'm clueless on this.
• Prepare newer constraints: New draft versions have new constraints some of which dont have a constraint class in code yet. Providing implementation for those could help. Perhaps peeking at other implementation on how this has been done could be a good start?
• Discuss: Participate in the discussion and provide insights, knowledge and/or learnings

[dafeder]

The other major PHP implementation, Opis, only supports down to draft-06. But it does support a few different drafts that are more recent, @handrews do you know are 06 and later backwards compatible or do they have breaking changes too?

This python implementation does support from 03 up to 2020-12, will try to take a closer look at the logic soon.

[handrews]

@dafeder draft-07 is strictly backwards compatible with draft-06, and is far more widely used. It is probably the most common draft at this point. Draft-07 is used in AsyncAPI and the W3C's Web of Things work, among other places.

Drafts 2019-09 and 2020-12 are not backwards compatible. Of the two, 2020-12 is far more broadly used, as (despite the 2 year gap) it came out before 2019-09 was widely adopted. 2020-12 is also part OpenAPI 3.1 (and higher, once we get 3.2 out the door later this year- I mostly work on OpenAPI rather than JSON Schema these days).