Draft: Extend FindingEntity with matches_auto_dismiss_policy

What does this MR do and why?

Extend FindingEntity with matches_auto_dismiss_policy.

To display a badge for findings in the MR security widget that will be automatically dismissed by a auto-dismiss policy, we need to include a new attribute in the FindingEntity.

• Add PolicyAutoDismissalChecker
• Add preload_auto_dismissal_checks which checks batches of findings
• Call preload_auto_dismissal_checks from get_report to prevent N+1
• Add a file method on the Security::Finding to match the signature of Vulnerabilities::Finding

References

• Frontend changes added in a follow-up: !216179

Screenshots or screen recordings

With FE changes from a follow-up MR:

Before	After

How to set up and validate locally

1. Enable the feature flag auto_dismiss_vulnerability_policies
2. Create a project
3. Create an auto-dismiss policy:

   vulnerability_management_policy:
   - name: Auto-dismiss acceptable secrets
     description: Auto-dismiss secrets
     enabled: true
     actions:
     - type: auto_dismiss
       dismissal_reason: not_applicable
     rules:
     - type: detected
       criteria:
       - type: file_path
         value: ".env"

4. Add .gitlab-ci.yml with secret detection

   include:
   - template: Jobs/Secret-Detection.latest.gitlab-ci.yml    

5. Add two secrets in a new MR:
   1. .env:

      AWS_TOKEN=AKIAZYONPI3G4JNCCWGQ

   2. .env2:

      AWS_TOKEN=AKIAZYONPI3G4JNCCWGA

6. Wait for the pipeline to finish
7. Inspect the network requests and open the response for merge_requests/:id/security_reports. Verify that the added findings have the attribute matches_auto_dismiss_policy correctly set.

MR acceptance checklist

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Related to #581394