Implement vulnerability counts in security MR widget for non-Ultimate users

This issue tracks making the vulnerability counts, as displayed on the security MR widget for Ultimate users, visible for non-Ultimate users.

Due to &4388 (comment 441836180), this issue will not make this functionality available to Core/FOSS. That will be tracked in #287784.

Implementation plan

• Move SAST and Secret Detection Vuex modules from EE to FOSS
• Move security summary component and utils from EE to FOSS
• Expose the sast_comparison_path and secret_scanning_comparison_path endpoints in FOSS rather than EE
• Create a new Vuex store for FOSS security MR widget, which consumes the above modules
• Add loading/error/success states in FOSS MR widget
  • The loading state cannot be implemented until #273424 (closed) is implemented. Actually, since fetching the counts is what needs a loading status, this has been done.
• Render counts using the SecuritySummary component
• Make sure the widget looks okay in dark mode: #281020 (comment 445893250)