Introducing GitLab browser-based active checks in DAST

Introduced browser-based active checks for DAST, starting with path traversal vulnerability detection (GitLab check 22.1) to replace ZAP alert 6. This enables more effective detection of vulnerabilities in modern web applications. Active checks are defined in YAML and can include match response, timing, and callback attack types. A worked example demonstrates how path traversal attacks are constructed and detected.