New v3 API endpoints for MFA

We have introduced a set of Multi-Factor Authentication (MFA) API endpoints. There are two stages in the BILL MFA process.

1. MFA setup: Add a valid phone number for setting up MFA in the organization (with POST /v3/mfa/setup), and then complete the setup by validating the entered phone number (with POST /v3/mfa/validate). At this point, the phone number is registered for the MFA sign in operation.

2. MFA sign in: Generate an MFA challenge (with POST /v3/mfa/challenge), and then complete MFA sign in by validating the MFA challenge (with POST /v3/mfa/challenge/validate). At this point, your API session is MFA trusted.