Add global cache config entry to new CI config

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

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

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

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

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

spec/lib/ci/gitlab_ci_yaml_processor_spec.rb

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

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

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

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

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

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

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

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

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

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