User ID: Ability to store user IDs in cookie and localStorage simultaneously

[carlosfelix]

🏷 Type of documentation

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

📋 Checklist

• [ ] Related pull requests in prebid.js or server are linked -> https://github.com/prebid/Prebid.js/pull/11482

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[ncolletti]

@ChrisHuie FYI this was merged in before the associating prebid.js PR was ready