Display a group by dropdown

What does this MR do and why?

This MR does two things:

• Introduce :vulnerability_report_grouping feature flag
• Display a listbox on the project vulnerability report when the feature flag is enabled

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable :vulnerability_report_grouping feature flag: echo "Feature.enable(:vulnerability_report_grouping)" | rails c
2. Fork https://gitlab.com/gitlab-examples/security/security-reports
3. Run a pipeline against the default branch
4. Visit Secure > Vulnerability Report
5. See the group by button

It currently does nothing other than setting the state. Upcoming work will introduce more functionality.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #420055 (closed)

config/feature_flags/development/vulnerability_report_grouping.yml

+---
+name: vulnerability_report_grouping
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/129709
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/422509
+milestone: '16.4'
+type: development
+group: group::threat insights
+default_enabled: false