Introduce response models for operations with application/json payload

[cliffano]

All operations currently pass all responses as string. While this is useful for xml, html, and plain text payload where processing is handled at the next layer (e.g. ruby_aem), this is not necessarily true for json payload where responses can be modelled at OpenAPI spec.

When the API introduces the model, need to also update ruby_aem to handle the model instead of json parsing.