Implement authentication for the new Generic Packages API

This builds the foundation for the new Generic Packages API endpoionts.
Authentication with personal access token and job token is supported.

It's using mock `GET ping` endpoint that will be replaced with a real
one in a following MR.

This new API is behind `generic_packages` feature flag.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/235490.

config/feature_flags/development/generic_packages.yml

+---
+name: generic_packages
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40045
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/239133
+group: group::release management
+type: development
+default_enabled: false

lib/api/api.rb

@@ -197,6 +197,7 @@ module API
       mount ::API::ConanPackages
       mount ::API::MavenPackages
       mount ::API::NpmPackages
+      mount ::API::GenericPackages
       mount ::API::GoProxy
       mount ::API::Pages
       mount ::API::PagesDomains

lib/api/generic_packages.rb

+# frozen_string_literal: true
+
+module API
+  class GenericPackages < Grape::API::Instance
+    before do
+      require_packages_enabled!
+      authenticate!
+
+      require_generic_packages_available!
+    end
+
+    params do
+      requires :id, type: String, desc: 'The ID of a project'
+    end
+
+    resource :projects, requirements: API::NAMESPACE_OR_PROJECT_REQUIREMENTS do
+      route_setting :authentication, job_token_allowed: true
+
+      namespace ':id/packages/generic' do
+        get 'ping' do
+          :pong
+        end
+      end
+    end
+
+    helpers do
+      include ::API::Helpers::PackagesHelpers
+
+      def require_generic_packages_available!
+        not_found! unless Feature.enabled?(:generic_packages, user_project)
+      end
+    end
+  end
+end

spec/requests/api/conan_packages_spec.rb

 # frozen_string_literal: true
+
 require 'spec_helper'
 
 RSpec.describe API::ConanPackages do

spec/requests/api/generic_packages_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe API::GenericPackages do
+  let_it_be(:personal_access_token) { create(:personal_access_token) }
+  let_it_be(:project) { create(:project) }
+
+  describe 'GET /api/v4/projects/:id/packages/generic/ping' do
+    let(:user) { personal_access_token.user }
+    let(:auth_token) { personal_access_token.token }
+
+    before do
+      project.add_developer(user)
+    end
+
+    context 'packages feature is disabled' do
+      it 'responds with 404 Not Found' do
+        stub_packages_setting(enabled: false)
+
+        ping(personal_access_token: auth_token)
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+
+    context 'generic_packages feature flag is disabled' do
+      it 'responds with 404 Not Found' do
+        stub_feature_flags(generic_packages: false)
+
+        ping(personal_access_token: auth_token)
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+
+    context 'generic_packages feature flag is enabled' do
+      before do
+        stub_feature_flags(generic_packages: true)
+      end
+
+      context 'authenticating using personal access token' do
+        it 'responds with 200 OK when valid personal access token is provided' do
+          ping(personal_access_token: auth_token)
+
+          expect(response).to have_gitlab_http_status(:ok)
+        end
+
+        it 'responds with 401 Unauthorized when invalid personal access token provided' do
+          ping(personal_access_token: 'invalid-token')
+
+          expect(response).to have_gitlab_http_status(:unauthorized)
+        end
+      end
+
+      context 'authenticating using job token' do
+        it 'responds with 200 OK when valid job token is provided' do
+          job_token = create(:ci_build, user: user).token
+
+          ping(job_token: job_token)
+
+          expect(response).to have_gitlab_http_status(:ok)
+        end
+
+        it 'responds with 401 Unauthorized when invalid job token provided' do
+          ping(job_token: 'invalid-token')
+
+          expect(response).to have_gitlab_http_status(:unauthorized)
+        end
+      end
+    end
+
+    def ping(personal_access_token: nil, job_token: nil)
+      headers = {
+        Gitlab::Auth::AuthFinders::PRIVATE_TOKEN_HEADER => personal_access_token.presence,
+        Gitlab::Auth::AuthFinders::JOB_TOKEN_HEADER => job_token.presence
+      }.compact
+
+      get api('/projects/%d/packages/generic/ping' % project.id), headers: headers
+    end
+  end
+end