New Adapter: Flatads

[flatads]

🏷 Type of documentation

• [x] new bid adapter
• [ ] update bid adapter
• [ ] new feature
• [ ] text edit only (wording, typos)
• [ ] bugfix (code examples)
• [ ] new examples

📋 Checklist

• [x] Related pull requests in prebid.js or server are linked -> Paste link in this list or reference it on the PR itself
• [x] For new adapters check submitting your adapter docs

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[flatads]

prebid-server new adapter PR