@cache: normaliser for vary= headers to bound the number of entries

[allen0099]

Problem

@cache(vary=[...]) (#268, PR #309) keys the cache on each listed request header's raw value, lower-cased name and trimmed value only. Header values come from the client, so every distinct value becomes its own entry: Accept-Language: en-US,en;q=0.9 and Accept-Language: en are two entries even when the route only supports en and zh-TW. Reducing a header to the values the route actually serves currently needs a custom key_builder that calls build_cache_key (#264) and leaves out vary= for that header. The docs show this in "Varying on request headers".

Proposal

Let vary also accept a mapping from header name to a normaliser:

def locale(value: str) -> str:
    return "zh-TW" if value.lower().startswith("zh") else "en"

@cache(ttl=60, vary={"Accept-Language": locale})

• The normaliser receives the trimmed value (empty when missing) and returns the string that goes into the key. It could also return an int, as build_cache_key accepts.
• The list form keeps working and keeps its keys. A header mapped to None behaves like the list form.
• The Vary response header is unchanged: a downstream cache still sees the raw header.
• invalidate(vary=...) must apply the same normaliser so it deletes the right entry.
• An allow-list ({"Accept": ["application/json", "text/html"]}, with anything else mapping to a default) could be sugar on top. Decide whether it is worth a second form.

Additive. Routes without a mapping keep their keys.