Add gitlab_shell_upload_pack_sidechannel feature flag

This feature flag sets the `use_sidechannel` field to true or false in
the response from POST /api/v4/internal/allowed.

This implements
https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1515 and
is part of https://gitlab.com/groups/gitlab-com/gl-infra/-/epics/652.

config/feature_flags/development/gitlab_shell_upload_pack_sidechannel.yml

+---
+name: gitlab_shell_upload_pack_sidechannel
+introduced_by_url: 
+rollout_issue_url:
+milestone: '14.8'
+type: development
+group: group::scalability
+default_enabled: false

lib/api/helpers/internal_helpers.rb

@@ -124,7 +124,8 @@ module API
           repository: repository.gitaly_repository.to_h,
           address: Gitlab::GitalyClient.address(repository.shard),
           token: Gitlab::GitalyClient.token(repository.shard),
-          features: Feature::Gitaly.server_feature_flags(repository.project)
+          features: Feature::Gitaly.server_feature_flags(repository.project),
+          use_sidechannel: Feature.enabled?(:gitlab_shell_upload_pack_sidechannel, repository.project, default_enabled: :yaml)
         }
       end
     end

spec/requests/api/internal/base_spec.rb

@@ -612,6 +612,30 @@ RSpec.describe API::Internal::Base do
             expect(json_response["gitaly"]["features"]).to eq('gitaly-feature-mep-mep' => 'false')
           end
         end
+
+        context "with a sidechannels enabled for a project" do
+          before do
+            stub_feature_flags(gitlab_shell_upload_pack_sidechannel: project)
+          end
+
+          it "has the use_sidechannel field set to true for that project" do
+            pull(key, project)
+
+            expect(response).to have_gitlab_http_status(:ok)
+            expect(json_response["gl_repository"]).to eq("project-#{project.id}")
+            expect(json_response["gitaly"]["use_sidechannel"]).to eq(true)
+          end
+
+          it "has the use_sidechannel field set to false for other projects" do
+            other_project = create(:project, :public, :repository)
+
+            pull(key, other_project)
+
+            expect(response).to have_gitlab_http_status(:ok)
+            expect(json_response["gl_repository"]).to eq("project-#{other_project.id}")
+            expect(json_response["gitaly"]["use_sidechannel"]).to eq(false)
+          end
+        end
       end
 
       context "git push" do