Support Content-Digest header when formalised

## 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 https://github.com/apache/couchdb/pull/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: https://github.com/apache/couchdb/issues/4566 Remove content-md5 hash

Start by reading the Digest Fields RFC linked in the issue and determine the finalized requirements for Content-Digest and Repr-Digest request and response headers. Review related issue #4566 and pull request #4574 for the context around removing Content-MD5. Done means CouchDB supports the finalized headers within the RFC's stated limits.

Written by the indexing model from the issue text.