Add FortiAuthenticator as OTP method

What does this MR do?

Adds FortiAuthenticator as OTP validation method. It refactors existing OTP code validation to Users::ValidateOtpService. The feature is currently behind the forti_authenticator feature flag. If the feature flag is enabled for the user, the OTP verification goes through FortiAuthenticator instead of Devise. User flow is not changed in this MR, the same views are presented to the user as previously.

#212312 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation ([if required](https://docs.gitlab.com/ee/development/documentation/workflow.html#when-documentation-is-required))
• Code review guidelines
• Merge request performance guidelines
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• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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