Conformance classes in OpenAPI schema

[emmanuelmathot]

It would be useful to have the conformance classes directly specified in the schemas. We could use it to automatically generate the landing page in the case of a STAC API framework leveraging code generation.
It would be possible to specify them at schema or operation level for a fine granularity. For instance with item-search:

openapi: 3.0.3
info:
  title: STAC API - Item Search
  version: 1.0.0-rc.2
  description: >-
    This is an OpenAPI definition of the SpatioTemporal Asset Catalog API - Item Search
    specification.
  contact:
    name: STAC Specification
    url: 'http://stacspec.org'
  license:
    name: Apache License 2.0
    url: 'http://www.apache.org/licenses/LICENSE-2.0'
  conformance: [ "https://api.stacspec.org/v1.0.0-rc.2/item-search" ]
tags:
  - name: Item Search
    description: essential characteristics of a STAC API
paths:
[...]

[philvarner]

Additional fields have to start with x-. I'm adding a field with name x-conformance-classes.

[philvarner]

These were added to the Foundation specs. I've made a mental note to add it to the extensions (but not filed the ~8 issues against every extension that could use it)