Dependency Firewall: Block risky packages before the build

Introduces GitLab Dependency Firewall, a new capability that blocks packages based on defined policies (malicious status, vulnerability severity, license type, package age) before they are installed in a build. It offers warn/block modes, hierarchical policy inheritance, and pre-pull checks via CLI. The feature aims to prevent malicious, vulnerable, or non-compliant packages from entering the software supply chain, reducing risk and manual remediation efforts.