The server's API key (HMAC secret).
The URL path to scope the key to. Normalized the same way as computePathKey.
The key's expiry, as an epoch-millisecond number or a
string. Stringified via String(expiry) before hashing, so the numeric
and string forms of the same value produce identical keys.
A 32-character lower-case hex key.
Compute an outsider key with an expiry:
HMAC-SHA256(apiKey, normalizedPath + "|" + expiry), truncated to 32 hex characters.