Support RADIUS protocol for 2FA / OTP
### Problem to solve Customers in secure environments may be required to use the RADIUS protocol to obtain a one-time passcode. GitLab does not currently support this option. ### Target audience Product Manager ### Proposal Support RADIUS as an additional OTP protocol ### What does success look like, and how can we measure that? Customers who rely on RADIUS are able to integrate it into GitLab ### Links / references [ZenDesk ticket # 112269](https://gitlab.zendesk.com/agent/tickets/112269) (internal link) /cc @jeremy
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