Add authentication adapters for MFA:

• One Time Password (OTP)
• Time based OTP (TOTP) like Google Authenticator
• It has to be an option enabled by the user (during registration?)
• It has to be flexible to support/customize how the OTP is sent to the user (email, SMS, etc)

Start from the PR subimitted by @lampi87 (#30

There should be clear documentation on how to extend the adapter to support customized MFA flows