Verify webhook requests

<timestamp>.<raw request body>

import { createHmac, timingSafeEqual } from "node:crypto";

export function verifySignalWebhook(
  rawBody: string,
  timestamp: string,
  providedSignature: string,
  secret: string,
) {
  const expected = "v1=" + createHmac("sha256", secret)
    .update(timestamp + "." + rawBody)
    .digest("hex");

  const provided = Buffer.from(providedSignature);
  const calculated = Buffer.from(expected);
  return provided.length === calculated.length
    && timingSafeEqual(provided, calculated);
}