Add group based Maven API endpoint

What does this MR do?

Add group based Maven API endpoint.

User will be able to download any maven package within the group with URL like /api/v4/groups/GROUP_ID_OR_NAME/-/packages/maven

Same as https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8274 but for group level

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/5839

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.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer