Introduce API for serving the artifacts archive

Add API to download build artifacts archive in context of GitLab API

/cc @DouweM @grzesiek

spec/models/build_spec.rb

@@ -538,7 +538,7 @@ def create_mr(build, commit, factory: :merge_request, created_at: Time.now)
     end
 
     context 'build is erasable' do
-      let!(:build) { create(:ci_build_with_trace, :success, :artifacts) }
+      let!(:build) { create(:ci_build, :trace, :success, :artifacts) }
 
       describe '#erase' do
         before { build.erase(erased_by: user) }
@@ -570,7 +570,7 @@ def create_mr(build, commit, factory: :merge_request, created_at: Time.now)
       end
 
       describe '#erased?' do
-        let!(:build) { create(:ci_build_with_trace, :success, :artifacts) }
+        let!(:build) { create(:ci_build, :trace, :success, :artifacts) }
         subject { build.erased? }
 
         context 'build has not been erased' do