Draft: Migrate active scan out of ZAP scripts

What does this MR do?

This MR migrates the logic for running an active scan out of the ZAP scripts.

• Adds services.ActiveScan to contain active scan logic
• Adds run_active_scan, active_scan_progress, and active_scan_status methods to ZAProxy
• Makes active_policy method on ActiveScanPolicy public so it can be used by ActiveScan
• Renames TargetWebsite to Target and updates it to handle API specification targets. It is used in ActiveScan and other classes to avoid having to check whether we're running an API scan in multiple locations

What are the relevant issue numbers?

gitlab-org/gitlab#218810 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer