On serialization

First thanks for working on a Golang project that understands/parses/validates multiple API specs!

I am using gnostic to parse & validate OpenAPIv3 specs (and later to validate API responses).
When translating from Gnostic's generated openapiv3.proto to my own intermediary format I noticed that some values have dubious defaults, such as OpenAPI's schema's `minimum` field:

https://github.com/googleapis/gnostic/blob/master/OpenAPIv3/OpenAPIv3.proto#L549

https://github.com/googleapis/gnostic/blob/master/OpenAPIv3/OpenAPIv3.go#L8100-L8102

IMO a boolean `has_minimum` should be introduced here, as a minimum of 0 is meaningful for real numbers.

Another note: it seems that OAS-flavored JSON schemas are not validated. They can contain `type: 'blipblop'` without issue.
Did I miss something? Is this planned to be supported?

I'm happy to open PRs!

I would also be very happy to see other Golang libs you like that
* parse at least OpenAPI specs
* validate the full spec (including OAS-flavored schemas), displaying useful warnings/errors
* can validate a net/http response against a spec

I'm building https://fuzzymonkey.co's client in Go but really the most complete libs seem to be in JS...