Update signature process

[DenisCarriere]

Should only need to replace this line

https://github.com/pinax-network/substreams-sink-websockets/blob/3141631738bfc135262ed8c066efd8d30c5bef94/src/fetch/POST.ts#L29C22-L30C1

Before

const msg = Buffer.from(timestamp + body);

After

const expiry = req.headers.get("x-signature-expiry");
const msg = Buffer.from(expiry);

[DenisCarriere]

Implemented