Multi-value header matching API

While the mechanics of plumbing multiple-valued headers to matchers have been taken care of by https://github.com/envoyproxy/envoy/security/advisories/GHSA-2v25-cjjq-5f4w and its aftermath, we still do not have a particularly ergonomic way to match on multiple header values. We now have a canonicalized flat string of all header values, which requires gymnastics with regular expressions etc. to do a multi-value match.

Ideally we have a match interface that is capable of expressing first-class matchers that operate on a multimap, e.g. the ability to model multiple matchers and apply semantics to match such as "any of" or "all of".