Exposes the property `approval_rules_overwritten`

For the `approvals_settings` response.
It was needed because we automatically populate the MR `rules` from
the project if necessary, but there was no way for the edit UI to know
if these were project rules or MR rules.

ee/lib/ee/api/entities.rb

@@ -263,6 +263,10 @@ module EE
 
       # Decorates ApprovalState
       class MergeRequestApprovalRules < Grape::Entity
+        expose :approval_rules_overwritten do |approval_state|
+          approval_state.approval_rules_overwritten?
+        end
+
         expose :wrapped_approval_rules, as: :rules, using: MergeRequestApprovalRule
         expose :overall_approvals_required, as: :fallback_approvals_required
       end