Add a RuboCop to prevent new not_owned endpoints

I noticed that we still sometimes added these, so I figured a RuboCop
was the easiest way to prevent that in the future

rubocop/cop/gitlab/avoid_feature_category_not_owned.rb

+# frozen_string_literal: true
+
+require_relative '../../code_reuse_helpers'
+
+module RuboCop
+  module Cop
+    module Gitlab
+      class AvoidFeatureCategoryNotOwned < RuboCop::Cop::Cop
+        include ::RuboCop::CodeReuseHelpers
+
+        MSG = 'Avoid adding new endpoints with `feature_category :not_owned`. See https://docs.gitlab.com/ee/development/feature_categorization'
+
+        def_node_search :feature_category_not_owned?, <<~PATTERN
+          (send nil? :feature_category (sym :not_owned) $...)
+        PATTERN
+
+        def_node_search :feature_category_not_owned_api?, <<~PATTERN
+          (pair (sym :feature_category) (sym :not_owned))
+        PATTERN
+
+        def on_send(node)
+          return unless file_needs_feature_category?(node)
+          return unless setting_not_owned?(node)
+
+          add_offense(node, location: :expression)
+        end
+
+        private
+
+        def file_needs_feature_category?(node)
+          in_controller?(node) || in_worker?(node) || in_api?(node)
+        end
+
+        def setting_not_owned?(node)
+          feature_category_not_owned?(node) || feature_category_not_owned_api?(node)
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/gitlab/avoid_feature_category_not_owned_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require_relative '../../../../rubocop/cop/gitlab/avoid_feature_category_not_owned'
+
+RSpec.describe RuboCop::Cop::Gitlab::AvoidFeatureCategoryNotOwned do
+  subject(:cop) { described_class.new }
+
+  shared_examples 'defining feature category on a class' do
+    it 'flags a method call on a class' do
+      expect_offense(<<~SOURCE)
+        feature_category :not_owned
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Avoid adding new endpoints with `feature_category :not_owned`. See https://docs.gitlab.com/ee/development/feature_categorization
+      SOURCE
+    end
+
+    it 'flags a method call on a class with an array passed' do
+      expect_offense(<<~SOURCE)
+        feature_category :not_owned, [:index, :edit]
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Avoid adding new endpoints with `feature_category :not_owned`. See https://docs.gitlab.com/ee/development/feature_categorization
+      SOURCE
+    end
+  end
+
+  context 'in controllers' do
+    before do
+      allow(subject).to receive(:in_controller?).and_return(true)
+    end
+
+    it_behaves_like 'defining feature category on a class'
+  end
+
+  context 'in workers' do
+    before do
+      allow(subject).to receive(:in_worker?).and_return(true)
+    end
+
+    it_behaves_like 'defining feature category on a class'
+  end
+
+  context 'for grape endpoints' do
+    before do
+      expect(subject).to receive(:in_api?).and_return(true)
+    end
+
+    it_behaves_like 'defining feature category on a class'
+
+    it 'flags when passed as a hash for a Grape endpoint as keyword args' do
+      expect_offense(<<~SOURCE)
+        get :hello, feature_category: :not_owned
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Avoid adding new endpoints with `feature_category :not_owned`. See https://docs.gitlab.com/ee/development/feature_categorization
+      SOURCE
+    end
+
+    it 'flags when passed as a hash for a Grape endpoint in a hash' do
+      expect_offense(<<~SOURCE)
+        get :hello, { feature_category: :not_owned, urgency: :low}
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Avoid adding new endpoints with `feature_category :not_owned`. See https://docs.gitlab.com/ee/development/feature_categorization
+      SOURCE
+    end
+  end
+end