Add export button to project security dashboard

What does this MR do and why?

This is the start of this feature development. This MR just adds the export button (UI-only) on the project security dashboard.

The backend is still working on the endpoint. Up next, will be the API integration.

Button	Alert Banner

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable feature flag: vulnerabilities_pdf_export
2. Clone this project: https://gitlab.com/gitlab-examples/security/security-reports
3. Run a successful pipeline
4. Go to the project security dashboard
5. You will see the export button, and on clicks, it displays the alert

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #545959 (closed)