Extend Variable Migration Support with `isSecret` Flag for ADO's variable groups

Overview

Enhance the variable migration functionality by adding support for the isSecret flag when interacting with the Azure DevOps API. At the moment such variable is migrated as an empty key as ADO encrypts such variable at rest it's not exposed in the API or UI. One of the route would be to deal with it via pipeline (similar to #1180).

Tasks

• Update docs if needed (workflow, runbooks, README, migration metrics, etc.)

Acceptance Criteria

• Existing UTs pass locally
• 100% code coverage on new code

@gitlab-org/professional-services-automation/tools/migration