Introduce reachability filter

What does this MR do and why?

Adds the Reachability Filter behind a feature flag to the Project and Group level vulnerability reports.

References

Screenshots or screen recordings

How to set up and validate locally

1. Enable feature flag: echo "Feature.enable(:reachability_es_filter)" | rails c
2. Visit Project & Group Vulnerability Report
3. Filter by Reachability

Note: You'll need to have ES support on your local GDK. You can follow this guide.

MR acceptance checklist

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Related to #543346