- adds `allowed_path` to strong validate `LegacyReader`
  and `LegacyWriter`
- does not change any of todays behaviour for data serialization
- makes the interface for reader and writer to accept
  `importable/exportable_path` that will be used by `ndjson`
  to store data in separate files
- moves consuming of attributes outside of the relation tree restorer
  to ease the way how we process them
- splits specs for `LegacyReader::User/File` and introduces
  shared example for that: this is also the majority of changes
  introduced by this MR in order to move them
- slightly code-formats and re-organises group and project
  import/export