Support 3.1 vocabulary for applying subschemas

[charjr]

Vocabulary can be found here: https://json-schema.org/draft/2020-12/json-schema-core#section-10

Keywords such as if, which are not present in the 3.0 vocabulary, are left untouched as arrays.

To support 3.1 these keywords should be turned into Schema objects.

Since OpenAPI 3.0 only supports custom keywords that are prefixed with "x-" I think it should be safe to support these keywords without breaking backwards compatibility.