Adaptive MFA video walk-through

Video walk-through of adaptive MFA with Authsignal rules, plus how to reproduce the flow using Postman and the Server API.

To follow along with Postman, see API testing using Postman for importing the Server API from OpenAPI and setting baseUrl, secretKey, and userId.This walk-through uses these Server API endpoints:

Enroll verified authenticator — POST /users/{userId}/authenticators
Track action — POST /users/{userId}/actions/{action}
Validate action — POST /validate
Get action — GET /users/{userId}/actions/{action}/{idempotencyKey}