Persist partial_scan_mode when storing scans

What does this MR do and why?

This MR adds a new vulnerability_partial_scans table to persist partial scanning information.

References

• Implementation issue: Persist GLAS diff-based scan in rails backend (#543636 - closed)
• Epic: &17758 (closed)
• Architecture design: https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/sast_glas_diff_scan/

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the feature flag: echo 'Feature.enable(:vulnerability_partial_scans)' | bundle exec rails c

2. Create a new project with this .gitlab-ci.yml:

   sast:
     stage: test
     script: wget https://gitlab.com/gitlab-org/gitlab/-/raw/5d7bc4e75ae688b968a6debc0b8f62e7eb1f54d1/ee/spec/fixtures/security_reports/master/gl-sast-report-differential.json
     artifacts:
       reports:
         sast: gl-sast-report-differential.json

3. After the pipeline completes, check that a partial scan record was created: Vulnerabilities::PartialScan.last

MR acceptance checklist

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