DAST Site validation - Create rollout feature flag

• parent issue: #233020 (closed)

Summary

This issue is for adding a feature flag that will enable us to roll out #238577 (closed) and #238578 (closed)

Design

• Site Validation design issue
• Creation site profile flow:
  • Figma file

Implementation Plan

• Create the security_on_demand_scans_site_validation feature flag.