feat(GlBanner): add button attributes to banner

mentioned in commit c20b06ee

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!155 (merged)

mentioned in merge request status-page!820 (merged)

mentioned in merge request gitlab!124927 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3432 (merged)

mentioned in merge request gitlab-docs!4012 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3479 (merged)