Compare results for scan_finding approval rules from multiple pipelines

What does this MR do and why?

This MR adds ability for Scan Finding Approval Rules (created from Scan Result Policies) to verify results from multiple pipelines related to given source branch and target branch SHA.

How to set up and validate locally

1. Create new project
2. Enable multi_pipeline_scan_result_policies feature for this project
3. Add .gitlab-ci.yml file that includes detached pipeline:

     container_scanning:
       image: "busybox:latest"
       stage: test
       allow_failure: true
       artifacts:
         reports:
           container_scanning: gl-container-scanning-report.json
         paths: [gl-container-scanning-report.json]
       dependencies: []
       script:
         - wget -O gl-container-scanning-report.json https://gitlab.com/-/snippets/2438327/raw/main/gl-container-scanning-report-empty.json?inline=false
       rules:
         - if: $CONTAINER_SCANNING_DISABLED
           when: never
         - if: $CI_COMMIT_BRANCH
   
     other_job:
       image: busybox:latest
       stage: test
       script:
         - echo "This is other branch"
   
     detachedjob:
       rules:
         - when: always
       script:
         - echo "Hello from detached job"

4. Add Scan Result Policy (go to Security & Compliance -> Policies), click New Policy, select Scan Result Policies and add new policy and merge created MR:

     name: Enforce check when there is at least 1 critical
     description: ''
     enabled: true
     actions:
     - type: require_approval
       approvals_required: 1
       group_approvers_ids:
       - # add your group ID
     rules:
     - type: scan_finding
       branches: []
       scanners: []
       vulnerabilities_allowed: 0
       severity_levels:
       - critical
       vulnerability_states:
       - newly_detected

5. Add new MR with change to .gitlab-ci.yml:

     diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
     index 0cf690cc8622c2262dd1ea492e4019e40dc078d0..46167e7f57632551d085397fe6994a596d4c1ae3 100644
     --- a/.gitlab-ci.yml
     +++ b/.gitlab-ci.yml
     @@ -31,7 +31,7 @@ container_scanning:
         paths: [gl-container-scanning-report.json]
       dependencies: []
       script:
     -    - wget -O gl-container-scanning-report.json http://gdk.test:3000/-/snippets/22/raw/main/gl-container-scanning-report-empty.json?inline=false
     +    - wget -O gl-container-scanning-report.json http://gdk.test:3000/-/snippets/22/raw/main/gl-container-scanning-report-1-critical.json?inline=false
       rules:
         - if: $CONTAINER_SCANNING_DISABLED
           when: never

6. Verify if approvals in the MR are required

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 #379108 (closed)