Instance level exportable security reports export logic

Why this MR?

Related issue: #213014 (closed)

Users have selected projects for their security dashboards and they want to be able to export the vulnerabilities for those projects as CSV files to share/print outside Gitlab.

What does this MR do?

This MR is introducing changes on existing project vulnerability export logic to make it possible to run the same logic for different type of models(exportables). Also while implementing the logic, I also introduced small improvements on existing implementation.

Since this MR is actually a bit out of size, I will introduce the API endpoints in another MR and will add the changelog entry for the feature in that MR.

I could actually create separate MRs for each commit but I thought that seeing the big picture would help the reviewer to understand the rationale behind the changes.

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