Draft: Add avatar icon for GitGuardian integration

What does this MR do and why?

Contributes to #601910

Problem

The GitGuardian integration settings page does not display a logo/icon.

Solution

Add a custom avatar_url method to Integrations::GitGuardian pointing to the gitguardian.svg asset from @gitlab/svgs.

References

• Issue: #601910
• SVG added in: gitlab-org/gitlab-services/design.gitlab.com!6009 (merged)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Ensure @gitlab/svgs is updated to a version containing gitguardian.svg.
2. Navigate to Project > Settings > Integrations.
3. Verify the GitGuardian integration displays the logo icon.

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.