feat: cleanup mfa logic in signer

[blakecduncan]

Pull Request Checklist

• Did you add new tests and confirm existing tests pass? (yarn test)
• Did you update relevant docs? (docs are found in the site folder, and guidelines for updating/adding docs can be found in the contribution guide)
• Do your commits follow the Conventional Commits standard?
• Does your PR title also follow the Conventional Commits standard?
• If you have a breaking change, is it correctly reflected in your commit message? (e.g. feat!: breaking change)
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• Did you follow the contribution guidelines?

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PR-Codex overview

This PR focuses on enhancing multi-factor authentication (MFA) handling in the codebase, introducing new error classes and functions to manage MFA requirements more effectively.

Detailed summary

• Added MfaRequiredError class to handle MFA requirements.
• Introduced parseMfaError function to parse MFA errors.
• Updated various files to replace multiFactor with multiFactors.
• Refactored authentication methods to support multiple MFA factors.
• Improved error handling in email authentication flow.

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