support for common resources

[alexander-fenster]

"Orphan" resources (those that do not have any messages related to them) may exist on file level.

option (google.api.resource_definition) = {
  type: "cloudresourcemanager.googleapis.com/Project"
  pattern: "projects/{project}"
};

We need to support them in the same way as we do for those within messages.

Related issues: googleapis/nodejs-translate#402, googleapis/nodejs-redis#243