Move API::Services constants to a helper

This moves various constants previously defined in API::Services to the
API::Helpers::ServicesHelpers module, using class methods instead of
constants. This allows EE to extend the returned values, without having
to modify the CE source code directly.

ee/lib/ee/api/helpers/services_helpers.rb

+# frozen_string_literal: true
+
+module EE
+  module API
+    module Helpers
+      module ServicesHelpers
+        extend ActiveSupport::Concern
+
+        class_methods do
+          extend ::Gitlab::Utils::Override
+
+          override :services
+          def services
+            super.merge(
+              'github' => [
+                {
+                  required: true,
+                  name: :token,
+                  type: String,
+                  desc: 'GitHub API token with repo:status OAuth scope'
+                },
+                {
+                  required: true,
+                  name: :repository_url,
+                  type: String,
+                  desc: "GitHub repository URL"
+                }
+              ],
+              'jenkins' => [
+                {
+                  required: true,
+                  name: :jenkins_url,
+                  type: String,
+                  desc: 'Jenkins root URL like https://jenkins.example.com'
+                },
+                {
+                  required: true,
+                  name: :project_name,
+                  type: String,
+                  desc: 'The URL-friendly project name. Example: my_project_name'
+                },
+                {
+                  required: false,
+                  name: :username,
+                  type: String,
+                  desc: 'A user with access to the Jenkins server, if applicable'
+                },
+                {
+                  required: false,
+                  name: :password,
+                  type: String,
+                  desc: 'The password of the user'
+                }
+              ],
+              'jenkins-deprecated' => [
+                {
+                  required: true,
+                  name: :project_url,
+                  type: String,
+                  desc: 'Jenkins project URL like http://jenkins.example.com/job/my-project/'
+                },
+                {
+                  required: false,
+                  name: :pass_unstable,
+                  type: ::API::Services::Boolean,
+                  desc: 'Multi-project setup enabled?'
+                },
+                {
+                  required: false,
+                  name: :multiproject_enabled,
+                  type: ::API::Services::Boolean,
+                  desc: 'Should unstable builds be treated as passing?'
+                }
+              ]
+            )
+          end
+
+          override :service_classes
+          def service_classes
+            [
+              ::GithubService,
+              ::JenkinsService,
+              ::JenkinsDeprecatedService,
+              *super
+            ]
+          end
+        end
+      end
+    end
+  end
+end