User Permission export backend changes

What does this MR do?

This MR adds the backend changes for User Permission export.

The api will be invoked when an administrator requests (#273496 (closed)) for User Permission export in the Users section of the Admin dashboard.

It internally invokes the export service that generates the CSV report.

Feature Flag

export_user_permissions_feature_flag, defaulted to false.

Please note, no Changelog has been included in this MR accordingly.

License

Premium and above

Mentions #273482 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required) (to be included in a separate doc MR)
• 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