Epics relantioship API

What does this MR do?

Add API endpoints for listing, adding and removing related epics.

What are the relevant issue numbers?

closes #7326 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• 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

ee/lib/api/helpers/epics_helpers.rb

@@ -27,9 +27,20 @@ def authorize_can_destroy!
       def epic
         @epic ||= user_group.epics.find_by(iid: params[:epic_iid])
       end
+
+      def find_epics(finder_params: {}, preload: nil)
+        args = declared_params.merge(finder_params)
+        args[:label_name] = args.delete(:labels)
+
+        epics = EpicsFinder.new(current_user, args).execute.preload(preload)
+
+        if args[:order_by] && args[:sort]
+          epics.reorder(args[:order_by] => args[:sort])
+        else
+          epics
+        end
+      end
       # rubocop: enable CodeReuse/ActiveRecord
     end
   end
 end
-
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