Add first custom validator for new ci config

This follows a standard `ActiveModel` pattern of creating a custom
validators. We use `ActiveModel::EachValidator` here that reuses methods
provided by `LegacyValidationHelpers`.

We will remove `LegacyValidationHelpers` on some point in the future, at
the later stages of CI configuration refactoring. It may be possible
to rewrite custom validators to use format like:

`validates :config, array_of: String`

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

@@ -14,15 +14,7 @@ module Gitlab
           include Validatable
 
           validations do
-            include LegacyValidationHelpers
-
-            validate :array_of_strings
-
-            def array_of_strings
-              unless validate_array_of_strings(self.config)
-                errors.add(:config, 'should be an array of strings')
-              end
-            end
+            validates :config, array_of_strings: true
           end
 
           def value

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

@@ -4,6 +4,7 @@ module Gitlab
       module Node
         class Validator < SimpleDelegator
           include ActiveModel::Validations
+          include Node::Validators
 
           def initialize(node)
             super(node)

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

@@ -0,0 +1,19 @@
+module Gitlab
+  module Ci
+    class Config
+      module Node
+        module Validators
+          class ArrayOfStringsValidator < ActiveModel::EachValidator
+            include LegacyValidationHelpers
+
+            def validate_each(record, attribute, value)
+              unless validate_array_of_strings(value)
+                record.errors.add(attribute, 'should be an array of strings')
+              end
+            end
+          end
+        end
+      end
+    end
+  end
+end