Create a debug module with join keys for all datasets

It's unclear for users what happens under the radar of the data_merge_module. Big part of it is join keys, primary keys and foreign keys of datasets, which are often set by our defaults and not visible otherwise than our code.

Task: to create a module, which would list the join_keys for all datasets and visualize the merging between different datasets.

Paging @Polkas who spent almost a few days debugging an issue which ultimately was resolved by explicitly passing `join_keys` (`cdisc_data` argument) or usage of cdisc_connectors.

Provenance:
```
Creator: kpagacz
```