enable duo auth as a 2fa provider

What does this MR do and why?

Enable cloud 2fa provider as an OTP option. Requested in #15760 (closed)

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Needs a working duo account and an set of duo integration keys: https://duo.com/docs/authapi https://duo.com/docs/authapi

once setup users will need to feature flag duo_auth enabled on them

after that. attempt to login as user. user with get asked for 2fa code (type push to trigger a push, or enter code from duo app) submit and hopefully be logged in

MR acceptance checklist

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