Add more metadata to ai_corpus for document export

When exporting a dataset we need to add more metadata.  We can use the ai_corpus datatype for this.  We need to add: dataset name, from/to date and perhaps model id.

We need to update all apis that use the data so change:

Operation, service and schema.

 For the start and end times, we will obtain them from a Cloud REST API. For now, and if not present, just do from the start until current time.