Challenge endpoint to request MFA (via Email). Call this endpoint to MFA via email when you receive MFA required error in the token endpoint.

Note: This endpoint should not be called to MFA via Authenticator app. In this case, the /token endpoint should be called using the MFA Example.

        {
           "challenge_type": "Email",
           "mfa_token": "{mfa token }",
           "user_token": "{user token}"
        }