Break out JSON Schema parser support and formalize

[EricWittmann]

It may be useful to refactor the current JSON Schema parsing support we have in OpenAPI (2.x and 3.x versions of the OAI spec define a customized subset of JSON Schema). We can simply have JSON Schema be another data model we support and then call out to it when we need to parse e.g. a datatype in OpenAPI or AsyncAPI. If we do this well we could then register it as a parser implementation with the JSON Schema upstream community.