Add support for `x-oas-draft-alternateSchemas`

OpenAPI lags behind JSON Schema, which means OpenAPI APIs do not benefit from the latest fixes and improvements to the latter.

For example:

1. JSON Schema Draft 4's patternProperties is not supported in OpenAPI, even though the latter is based on Draft 5 (OAI/OpenAPI-Specification#687).
2. additionalProperties: false is not compatible with OpenAPI's inheritance model using allOf [1]. This is blocking for strict validation of JSON request bodies that depend on inheritance (#837).
3. Similar issues exist when combining nullable, readOnly, and writeOnly with OpenAPI's inheritance model.

It would be nice if we could already use x-oas-draft-alternateSchemas to specify other schemas to address some of these issues, until alternativeSchema is available in OpenAPI 3.1 (https://github.com/OAI/OpenAPI-Specification/pull/1736).

Related issue: https://github.com/Rebilly/ReDoc/issues/482

[1] https://stackoverflow.com/questions/22689900/json-schema-allof-with-additionalproperties