@cache: vary= to key responses on selected request headers

[allen0099]

Problem

The key does not include any request header. A route whose response depends on Accept-Language, Accept or Accept-Encoding serves whichever variant was cached first to every client. Setting Vary on the response tells downstream caches, but not our own backend.

Proposal

Add @cache(vary=["Accept-Language"]):

• Append each listed header's value (normalised: lower-cased name, trimmed value, missing as empty) to the key, encoded with escape_key_component.
• Add the same names to the response's Vary header.
• Header values are client-controlled, so document that each listed header multiplies the number of entries. Consider an optional allow-list or normaliser per header, e.g. to reduce Accept-Language to a supported locale.

Additive; routes without vary keep their keys.