Make tesserocr vs easyocr configurable for Docling integration

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Docling defaults to using easyocr for optical character recognition, but we have some downstream consumers that will prefer to use Docling's tesserocr for OCR. We need to expose a way for users to influence which we use, as it requires code changes in our Docling integration to swap the OCR engine used.

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This will likely also imply we need to adjust our docling in requirements.txt to pull in docling[tesserocr] instead of docling. The tesserocr variant pulls in both tesserocr and easyocr, allowing us to swap between each with the single dependency.