Support Content-Digest header when formalised

[rnewson]

Summary

Support the Content-Digest / Repr-Digest headers when the Digest Fields RFC is formalised (https://datatracker.ietf.org/doc/draft-ietf-httpbis-digest-headers/)

Desired Behaviour

Users of CouchDB can use the new request and response headers to gain additional confidence that there was no in-flight corruption, subject to the limits declared in the RFC.

Possible Solution

N/A

Additional context

We removed support for the deprecated Content-MD5 header in #4574 as part of a series of changes to remove all use of MD5 from CouchDB. Until the Digest Fields RFC we have no replacement. At that time, consider adding support.

Related issue: #4566 Remove content-md5 hash