feat(magika): make magika an optional dependency

[SyedaAnshrahGillani]

Description:

This pull request makes the magika library an optional dependency in the markitdown package.
Previously, magika (and its dependency onnxruntime) was a mandatory installation, even for users who
did not utilize its functionality.

By making magika optional, users can now choose to install it only if they require its features,
leading to a smaller and more efficient installation for most users.

Changes:

• packages/markitdown/pyproject.toml: Moved onnxruntime into a new optional dependency group magika.
• packages/markitdown/src/markitdown/_markitdown.py:
  • Removed the direct import of magika.
  • Implemented a conditional import for magika within the MarkItDown class's init method.
  • Added a check in _get_stream_info_guesses to ensure magika is available before attempting to use
    it, gracefully skipping its functionality if not installed.

[SyedaAnshrahGillani]

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