feat: Add JSON and CSV importer/export

[hangy]

📝 Description

Adds import and export for the formats .json, .json.gz, and .csv. To do this without too much uselessly redundant code, some refactoring was done.

🔗 Related Issues

Contributes to #25

💡 Additional Notes

• Added a separate ETL class for .json.gz files, which just uses a gzip-stream
• JSON properties are shortened, as redundant field names in plain JSON files can add up to a lot of data.
• For CSV, a quick search didn't lead me to standardized format. This is just a suggestion to start with.
• Parquet and HDF5 will still need an implementation, but I didn't want to make this PR too big, and would have to research those formats a bit more.