proposal(http/unstable): support RFC 9530 Want-Content-Digest / Want-Repr-Digest fields #7266

#7238 adds an unstable @std/http/unstable-digest-fields module for creating and verifying RFC 9530 Content-Digest and Repr-Digest field values.

RFC 9530 §4 also defines the Want-Content-Digest and Want-Repr-Digest fields, which let a sender express a preference for which digest algorithms the peer should use (as an Integer Preference dictionary, e.g. Want-Repr-Digest: sha-512=3, sha-256=10). These were deliberately scoped out of #7238 to keep the initial module small.

This issue tracks adding helpers for creating and parsing the two Want-* fields, e.g.:

• serializing an algorithm-preference map to a field value
• parsing a received field value into an ordered preference list, ignoring unsupported algorithms per the RFC

Raised in review: #7238 (comment)