feat(GlFilteredSearch): Add suggestionsListClass to suggestions list

What does this MR do?

This MR adds a suggestionsListClass to the filter search suggestions dropdown. This way, we can easily add allow users to add custom CSS classes to the dropdown. We recently noticed chart tooltips getting into its way in several places, for example see gitlab#280811 (closed)

This issue can be solved by adding a custom CSS class with a high z-index to the dropdown.

Does this MR meet the acceptance criteria?

Conformity

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• GitLab UI's contributing guidlines.
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• Added the ~"component:*" label(s) if applicable.

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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