Backport generic methods to CE

What does this MR do?

Backport generic methods to CE fron EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8107#note_114707683

• Allow to fetch all pipelines for every projects in a group and its subgroups.
• Allow to fetch the latest pipeline id for each projects of a group and its subgroups.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/6709

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.