static SIGNING_KEY_CACHE: Lazy<DashMap<String, Vec<u8>>>Expand description
Process-wide cache for derived SigV4 signing keys.
The signing key only changes once per calendar day (it is keyed on
date + region + service + secret). Caching it eliminates the four
chained HMAC-SHA256 derivations that signing_key() would otherwise run
on every outgoing request.
Key format: "YYYYMMDD:{region}:{service}:{secret_key}"
Value: the 32-byte derived signing key.