Add ci config entry that represents array of paths

lib/gitlab/ci/config/node/paths.rb

+module Gitlab
+  module Ci
+    class Config
+      module Node
+        ##
+        # Entry that represents an array of paths.
+        #
+        class Paths < Entry
+          include Validatable
+
+          validations do
+            validates :config, array_of_strings: true
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/config/node/paths_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Ci::Config::Node::Paths do
+  let(:entry) { described_class.new(config) }
+
+  describe 'validations' do
+    context 'when entry config value is valid' do
+      let(:config) { ['some/file', 'some/path/'] }
+
+      describe '#value' do
+        it 'returns key value' do
+          expect(entry.value).to eq config
+        end
+      end
+
+      describe '#valid?' do
+        it 'is valid' do
+          expect(entry).to be_valid
+        end
+      end
+    end
+
+    context 'when entry value is not valid' do
+      let(:config) { [ 1 ] }
+
+      describe '#errors' do
+        it 'saves errors' do
+          expect(entry.errors)
+            .to include 'Paths config should be an array of strings'
+        end
+      end
+    end
+  end
+end