HMAC-SHA384 payload signing pattern (Bitfinex/Gemini-style).
Used by: Bitfinex, Gemini, and ~3 other exchanges.
Two variants selected via :variant config key:
Bitfinex (default): /api/path + nonce + body → HMAC-SHA384 → hex
Gemini: base64(json(payload)) → HMAC-SHA384 → hex, payload in header
Configuration
signing: %{
pattern: :hmac_sha384_payload,
variant: :bitfinex,
api_key_header: "bfx-apikey",
signature_header: "bfx-signature",
nonce_header: "bfx-nonce"
}