Feature: content/metadata file splitter

[Uinelj]

Since the new pipeline outputs a unique .jsonl file containing both textual content and metadata, working exclusively with textual data may be less comfortable:

1. Forced download of metadata that will be discarded
2. Forced usage of a jsonl-compatible parser in order to extract content

For now, we should provide a tool that splits up a given OSCAR Schema v2 file into a OSCAR Schema v1.2-compatible file.

[Uinelj]

As a first step, extract a v1.1-compatible file.