Adds simple breadcrumb comment to client class commentary including the
`google.api.api_version` value that this client was generated based on
and sends in its requests, as per AIP-4236. This breadcrumb will allow
client users to reference the API version used by the client in other
API artifacts e.g. product documentation.

Fixes internal tracking bug http://b/467138606, child of
http://b/467065424.