Feature flag: adding of use_core_delta_islands

Gitaly starts using a core delta island feature for pack reuse:
https://gitlab.com/gitlab-org/gitaly/-/merge_requests/1775
This flag will allow to switch between using them or not.

Part of: https://gitlab.com/gitlab-org/gitaly/issues/1519

lib/feature/gitaly.rb

@@ -10,6 +10,7 @@ class Feature
         cache_invalidator
         inforef_uploadpack_cache
         commit_without_batch_check
+        use_core_delta_islands
       ].freeze
 
     DEFAULT_ON_FLAGS = Set.new([]).freeze

spec/requests/api/internal/base_spec.rb

@@ -326,7 +326,7 @@ describe API::Internal::Base do
           expect(json_response["gitaly"]["repository"]["relative_path"]).to eq(project.repository.gitaly_repository.relative_path)
           expect(json_response["gitaly"]["address"]).to eq(Gitlab::GitalyClient.address(project.repository_storage))
           expect(json_response["gitaly"]["token"]).to eq(Gitlab::GitalyClient.token(project.repository_storage))
-          expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-cache-invalidator' => 'true', 'gitaly-feature-commit-without-batch-check' => 'true')
+          expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-cache-invalidator' => 'true', 'gitaly-feature-commit-without-batch-check' => 'true', 'gitaly-feature-use-core-delta-islands' => 'true')
           expect(user.reload.last_activity_on).to eql(Date.today)
         end
       end
@@ -346,7 +346,7 @@ describe API::Internal::Base do
             expect(json_response["gitaly"]["repository"]["relative_path"]).to eq(project.repository.gitaly_repository.relative_path)
             expect(json_response["gitaly"]["address"]).to eq(Gitlab::GitalyClient.address(project.repository_storage))
             expect(json_response["gitaly"]["token"]).to eq(Gitlab::GitalyClient.token(project.repository_storage))
-            expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-cache-invalidator' => 'true', 'gitaly-feature-commit-without-batch-check' => 'true')
+            expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-cache-invalidator' => 'true', 'gitaly-feature-commit-without-batch-check' => 'true', 'gitaly-feature-use-core-delta-islands' => 'true')
             expect(user.reload.last_activity_on).to be_nil
           end
         end
@@ -594,7 +594,7 @@ describe API::Internal::Base do
           expect(json_response["gitaly"]["repository"]["relative_path"]).to eq(project.repository.gitaly_repository.relative_path)
           expect(json_response["gitaly"]["address"]).to eq(Gitlab::GitalyClient.address(project.repository_storage))
           expect(json_response["gitaly"]["token"]).to eq(Gitlab::GitalyClient.token(project.repository_storage))
-          expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-cache-invalidator' => 'true', 'gitaly-feature-commit-without-batch-check' => 'true')
+          expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-inforef-uploadpack-cache' => 'true', 'gitaly-feature-cache-invalidator' => 'true', 'gitaly-feature-commit-without-batch-check' => 'true', 'gitaly-feature-use-core-delta-islands' => 'true')
         end
       end