feat: add factory for ocr engines via plugins

[dolfim-ibm]

Leverage factories and allow the usage of third-party packages as Docling plugins.

Using the setuptools extensions, third-party packages can implement and register models which can be selected in the Docling processing.

This is achieved by defining the following (example poetry syntax):

[tool.poetry.plugins."docling"]
"docling_ocr_engines" = "docling.models.plugins.ocr_engines"

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

[mergify]

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[dolfim-ibm]

@vagenas let's indeed make the models use __init__(*, ...) and have the factory adding the options= explicitly.