Schema negotiation with HTTP Prefer

[handrews]

Media type parameters are problematic due to requiring the media type to agree to and register the parameter. Media types such as application/json are reluctant to do so (e.g. refusing the consider the "profile" media type parameter despite an effort to get it into RFC 8259).

The HTTP Prefer header is a much more lightweight and flexible system for negotiating aspects of responses. We should investigate whether and how to add Prefer-based negotiation to the core spec.