Create a service method to fetch the comments on MRs / Issues

Summary

Call The GitLab API to retrieve the comments on an MR either per project or per group.

Checklist

• add a service method to retrieve all comments on one MR per project
• add a service method to retrieve all comments on one MR per group
• add a service method to retrieve all comments on one issue per project
• add a service method to retrieve all comments on one issue per group

Implementation details

• return from the method only the relevant data for displaying and filtering
• you can return a record for now