* Generalize the transfer logic so it is easy to build new transfer agents
This change moves code that reads shards and produces summaries into the
data package so that it can be reused to create new transfer agents,
similar to the BigQuery transfer agent in cron/internal/bq.
Signed-off-by: Caleb Brown <calebbrown@google.com>
* Lint fix and commentary.
Signed-off-by: Caleb Brown <calebbrown@google.com>
Signed-off-by: Caleb Brown <calebbrown@google.com>