Add offset alias property to be used in query_method.

Some clients require more traditional `offset`, `limit` functionality.  For example an implementation of a dojo Store on top of an endpoints API ([dojo-endpoints](/sstriker/dojo-endpoints)).  As this store needs to stick to a set API, it can't use the more efficient nextPageToken approach.

This patch adds an `offset` to `_EndpointsQueryInfo` and uses it when calling `fetch_page`.