Resolve "Create a service method to fetch the MRs per group and project"

Summary

This MR adds four methods for retrieving all merge requests per group and / or project from GitLab's REST API.

The methods allow for retrieving MRs that were created within a certain time stamp, as well as MRs that were last updated within the time stamp.

Checklist

All items on this checklist should be checked.

• I have added tests for the functionality that I added.
• I adhere to the style guide.

How to review

Changes are implemented in the files:

• src/main/java/nl/tudelft/gitbull/service/statistics/ProjectStatisticsService.java - method getCreatedProjectMergeRequests() and getUpdatedProjectMergeRequests()
• src/main/java/nl/tudelft/gitbull/service/statistics/GroupStatisticsService.java - method getCreatedGroupMergeRequests() and getUpdatedGroupMergeRequests()

Currently there is no UI functionality that reflects these changes.

Issues and tasks closed

Closes #80 (closed)