Token-authenticated client that pulls, transforms and stores data

Write a small program from scratch, one step at a time:

1. Get an access token through an OAuth flow.
2. Use the token to call a remote endpoint (an old database's API) and fetch its data.
3. Transform and parse the records into the shape you are asked for.
4. Write them to the destination, a new endpoint or a database, and compute a checksum to show nothing was lost on the way.