Provide the ability to CSV import comment based on the its visibility

Issue Summary

CSV import is not able to specify whether an issue's comment is private or not. All comments are added as public.

Steps to Reproduce

1. Try to export one issue which has a few comments restricted to the group in a CSV file.
   
2. Import the CSV file to a target project.
3. An issue with comments is imported successfully but the comments restriction is removed and all the comments in the issue are public.
   

   Expected Results

It should have the same comments restriction after the CSV import for the issue.

Actual Results

Private comments are visible to all and the comments restrictions are removed.

Workaround

Currently, there is no known workaround for this behavior. 

Previous Description:

At the moment, CSV importer only allows comments to be mapped to comment body and it is not possible to maintain the visibility of the comments. For example, the comment belongs to a Service Desk issue and it is an internal comment. After the import, the comment will be automatically set to public.

Thus, it would be great if CSV importer can provide an option to set the visibility of the comment during the mapping field process.