[WIP] Group level security reports export logic

Related issue

#213013 (closed)

What does this MR do?

This MR is introducing changes to make Vulnerabilities::Export model working with both Project and Group models. Schema changes seen in the diff will be merged into upstream/master in another MR(!29498 (merged)).

Basically the overall idea behind the change is, making sure Project and Group models implement the similar interfaces so we can use them transparently without exposing the type information to Export model and other service classes.

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