Implements Tipalti's SOAP API request signing scheme.

Per Tipalti's published authentication process, every SOAP call carries a signature (hmac/hashkey) computed as:

1. Build an authentication string from, in order: the payer name, the payee ID (idap, payee-function calls only), the current Unix timestamp (UTC), and the operation's EAT (Encryption Additional Terms) parameter, if that operation defines one.
2. UTF-8 encode that string.
3. HMAC-SHA256 the encoded string using the private API key, hex-encoded.

This module has no network or process dependencies — it's pure string/crypto logic, built entirely on the OTP :crypto application (no external dependency).