MAINT: JSONFileConverter can parse additonal valid JSON content

Summary

The JSONFileConverter creates a new level of children: one for the dict that was parsed before.

Focus

converters.py: Some refactoring in DictElementConverter and JSONFileConverter

Test Environment

unit and inttests

Check List for the Author

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• All automated tests pass
• Reference related issues https://gitlab.indiscale.com/caosdb/customers/leibniz-zmt/management/-/issues/145
• Up-to-date CHANGELOG.md (or not necessary)
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

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