DNSBench: Add version 1.3.668.0

[chortbauer]

New manifest for the freeware dns benchmark tool DNSBench

• [x] I have read the Contributing Guide.

[chortbauer]

/verify

[github-actions[bot]]

All changes look good.

Wait for review from human collaborators.

dnsbench

• [x] Description
• [x] License
• [x] Hashes

[rasa]

Copied from here. I have mentioned numerous times before that having one huge monolithic bucket is unsustainable. And taking apps from volunteers who've maintained them for years may dis-incentivize them from continuing to maintain the less popular apps.