feat: Extend the evaluators to support external predictions stored in files

[nikos-livathinos]

Support external predictions according to the description given in #112

• Extend all evaluators to support the optional parameter external_predictions_path.
• If such a path is provided, it is used to load DoclingDocument objects from files instead of the parquet dataset.
• GT is always taken from the parquet.
• The path can contain files with predicted DoclingDocuments in various formats (json, doctags, yaml).
• Update unit tests.
• Extend CLI for docling-eval evaluate:

--external-predictions-path        PATH            Path to load existing DoclingDocument predictions. The filename must follow the pattern [doc_id].[json|dt|yaml|yml]

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