Example src-stats.yaml for the CCHIC/OMOP use case

Hi everyone :eyes:

I was wondering whether you could provide a dummy `src-stats.yaml` file for the CCHIC/OMOP use case that is compatible with [the config file](https://github.com/alan-turing-institute/sqlsynthgen/blob/main/examples/cchic_omop/config.yaml). I'm trying to generate a mock OMOP dataset without access to real data.

PS: great work on the library, this is really cool :raised_hands:

Start by reading examples/cchic_omop/config.yaml and inspect how the project defines or loads src-stats.yaml. Create a compatible dummy file for the CCHIC/OMOP configuration, then run the repository's generation workflow and confirm it produces a mock OMOP dataset without real input data.

Written by the indexing model from the issue text.