Reintroduce lfs_link_existing_object feature flag

We have an issue when mirrors doesn't download new LFS files
due to this optimization

Let's reintroduce the feature flag until we figure out how to fix
the issue

Changelog: changed

app/services/projects/lfs_pointers/lfs_download_service.rb

@@ -22,7 +22,7 @@ module Projects
       def execute
         return unless project&.lfs_enabled? && lfs_download_object
         return error("LFS file with oid #{lfs_oid} has invalid attributes") unless lfs_download_object.valid?
-        return link_existing_lfs_object! if lfs_size > LARGE_FILE_SIZE && lfs_object
+        return link_existing_lfs_object! if Feature.enabled?(:lfs_link_existing_object, project, default_enabled: :yaml) && lfs_size > LARGE_FILE_SIZE && lfs_object
 
         wrap_download_errors do
           download_lfs_file!

config/feature_flags/development/lfs_link_existing_object.yml

+---
+name: lfs_link_existing_object
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/41770
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/249246
+group: group::source code
+type: development
+default_enabled: false

spec/services/projects/lfs_pointers/lfs_download_service_spec.rb

@@ -241,6 +241,18 @@ RSpec.describe Projects::LfsPointers::LfsDownloadService do
       context 'and first fragments are the same' do
         let(:lfs_content) { existing_lfs_object.file.read }
 
+        context 'when lfs_link_existing_object feature flag disabled' do
+          before do
+            stub_feature_flags(lfs_link_existing_object: false)
+          end
+
+          it 'does not call link_existing_lfs_object!' do
+            expect(subject).not_to receive(:link_existing_lfs_object!)
+
+            subject.execute
+          end
+        end
+
         it 'returns success' do
           expect(subject.execute).to eq({ status: :success })
         end