Proactive content negotiation by RFC 9110, section 12: the media type
and the language of a response, picked from the offers of a server by
the Accept and Accept-Language fields of a request.

The functions that pick take a field value and the offers in the
server's order of preference. A nil field value means that the client
has no preference, so the first offer wins. They return an offer as
given, or nil when none is acceptable. With a function that returns the
media type or language tag of an item, they pick items instead.

The negotiate function decides for a whole Ring request, with overrides
from query parameters and cookies, and returns the fields for Vary. The
functions after it make the headers and responses that follow.

A Pedestal interceptor that negotiates each request.

The interceptor is a map, so the library doesn't depend on Pedestal. A
route takes it as it is. Elsewhere, e.g. among the interceptors of a
service, pass it to io.pedestal.interceptor/interceptor first.

Ring middleware that negotiates each request before the handler gets
it.