Implement querying TSV files

[holtgrewe]

Is your feature request related to a problem? Please describe.
Having import (#1) and dump (#2) for TSV files is helpful but we need to have a good API for retrieving the information in Rust code.

Describe the solution you'd like
Implement such an API. As we are dealing with dynamic schemas, use serde_json::Value for loading the values into a JSON-like dict.

Describe alternatives you've considered
N/A

Additional context

• Implement import of TSV files #1
• Implement dump of TSV files #2