[BE] Use `path` param to return a drill down of code analytics

Discovery Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/12683

This is a further iteration of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14719

TODOs

• Include path param in CodeAnalyticsFinder
• Use it to search for file edits only in that path
• Validate path in controller
• Use path to build the hotspot tree