Write test migration script for us for initial DS migration test.

[tublitzed]

This can be rough for now, as we're testing a single sync user first. This will be for use with our initial test for migrating one sync user.

• Script should be able to point to existing Sync DB (dev is fine), and pull data by sync id (or some identifier that we can get with sync id).
• Should then be able to insert same data into Spanner dev DB.

As long as the DB hosts are easily configurable, we should be able to eventually modify this to run against stg/prod as we refine.

[tublitzed]

De-prioritized in favor of this one.