Store Security::Scan and Security::Finding entries

What does this MR do?

This MR changes the way we store Security::Scan and Security::Finding entries from after build finished hook to after pipeline finished hook. Also, for being able to mark duplicated findings, it's loading artifacts into memory one by one and not all at once which will improve the memory usage of the background jobs by reducing the number of objects created at once which will require way less vm_heap slots, therefore, less memory.

Fixes this MR provides;

1. Consistent sorting for all report types which is not the case with the current implementation as it's not sorting reports other than dependency_scanning ones which can cause missing feedback for the reports if there subsequent runs of the same build for the same pipeline.
2. These Security::Finding entries will be used to determine which report artifact(s) to download & parse while serving the response for vulnerability_findings REST API endpoint which has really bad response times at the moment(mostly timeouts).

Related to #238156

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team