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]

Merge Protections

Your pull request matches the following merge protections and will not be merged until they are valid.

🟢 Enforce conventional commit

Wonderful, this rule succeeded.

Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?: