S3 expect step

Need an expect step to match file with S3:

```
- expect:
compare:
postgres:
config: '{{ s3_config }}'
data:
bucket/file: expect.csv
```

The issue names no files, tests, or entry points. Start by locating the existing expect and compare implementations, then inspect how the repository handles the shown postgres configuration and data mapping. Done means the expect step can compare an S3 bucket/file object with expect.csv using the provided configuration, with tests covering the behavior.

Written by the indexing model from the issue text.