Verify signature against new payload

[ChristianPavilonis]

Need to adjust to work with these changes: IABTechLab/trusted-server#220

Verification Steps

1. Reconstruct the payload using values from the request:
   payload = f"{kid}:{request_host}:{request_scheme}:{request.id}:{ts}"

2. Verify the signature using the public key associated with kid

3. Validate the timestamp for replay protection:

   • Check ts is within acceptable window (e.g., not older than 5 minutes)
   • Optionally check it's not in the future

4. Validate domain binding (optional but recommended):

   • Verify request_host matches the expected publisher domain
   • Verify request_scheme is https (if required)