A signing GenerateKey task should indicate which asset it created

When creating a GenerateKey signing work request, there is no indication which asset was created. There is a debusine:work-request-debug-logs artifact, but it has no metadata and simply says 1 (which is not the asset id). The created asset has a work request id associated in the database, but debusine-admin asset list does not display it. As such connecting the GenerateKey work request to the created asset is a non-trivial affair.