Introduce BatchMerger service to merge MR in batches

What does this MR do?

Introduces ReleaseTools::Security::MergeRequestsBatchMerger to merge MRs in batches. This class does the following:

1. Fetches security implementation issues from the latest security release tracking issue.
2. For every security implementation issue Security::ImplementationIssue:
   • Verifies if their merge requests are ready to be processed. If they're not, it skips to the next one.
   • Validates their merge requests
   • If all of their merge requests are valid, proceed to merge them.
   • If any of them is invalid, it assigns them back to the author and creates a discussion on the merge request targeting master
3. If all the merge requests were merged, it cherry-picks the one targeting master in the current auto-deploy branch.
4. Notifies the result to Slack (with IssueResult)

This class is executed from MergeRequestsMerger if merge_in_batches feature is enabled.

To do (backend)

• Find security issues and merge requests through ReleaseTools::Security::IssueCrawler
• Log how many security issues were found
• Validate merge requests per issue
• If all merge requests are valid, merge them.
• Send the result to Slack
• Pick the merge request targeting master to the auto-deploy branch.
• Add specs

Related to https://gitlab.com/gitlab-org/release-tools/-/issues/369