Backend changes for user permission export

What does this MR do?

This MR adds the backend capabilities for an administrator of an instance to download the User Permissions report.

The workflow is as below:

1. API to create a User Permission Export (Admin::UserPermissionExportsController#create). This API will be invoked when a an admin requests for an export (#273496 (closed)) in the Users section of the admin dashboard. The API does the following:
   • Invokes a CreateService that created a user_permission_export_upload record for the admin user. Schedules a job to generate the report
2. The job executes the ExportService that generates the csv data, sends an email with the download link of the report and schedules a export deletion job after an hour
3. The download api fetches and returns the csv file requested by the admin user
4. The export deletion job scheduled to be run after an hour of the report creation, deletes the user_permission_export_upload record

Feature Flag

export_user_permissions_feature_flag, defaulted to false

License

Premium and above

Mentions #273482 (closed)

Screenshots (strongly suggested)

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