Add: MFA

[shrunyan]

The SDK needs to allow for Zesty.io authentication with Multi-Factor(a.k.a Two-factor) authentication. This will need to use the zesty auth service(https://auth-api.zesty.org/) to complete the flow. As context; zesty auth uses authy.com for providing MFA functionality.

Please write out the MFA flow and describe the necessary code changes. We want to ensure we understand how the flow will work before we proceed building it.