Added an expiration time as an RFC 3339 date-and-time string and as a UNIX timestamp to DynamoDB. I added the RFC 3339 version to preserve precision, since UNIX timestamps only go to the second. Open to removing it, though.
The UNIX timestamp lets us set a TTL on the DDB table with Terraform (see next diff). DynamoDB will then remove expired nonces, saving us from having to implement a cron job ourselves.
Depends on D8825