Insert entries into dedup cache with full SHA-256 hash and signature

[lukevalenta]

See https://groups.google.com/a/chromium.org/g/ct-policy/c/mUZSWkS3i8M/m/-60vvtV0AgAJ and https://github.com/FiloSottile/sunlight/releases/tag/v0.8.1 for context.