feat(gcs): implement GCS publish endpoint support and update config/show/system tests

system/t02_config/ConfigShowYAMLTest_gold

@@ -33,6 +33,7 @@ skip_contents_publishing: false
 skip_bz2_publishing: false
 filesystem_publish_endpoints: {}
 s3_publish_endpoints: {}
+gcs_publish_endpoints: {}
 swift_publish_endpoints: {}
 azure_publish_endpoints: {}
 packagepool_storage: {}