Enhance Multi-Processing for Migration in ADO

Overview

Refactor the migration functionality to improve multi-processing by running export and import operations within a single process. Transition from the current approach of parallelising all group/project exports followed by imports to a more streamlined and efficient process-based execution. Initial discussion started in this MR.

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