[core] Migrate frontends/ir/src/ to std::filesystem::path

[almilosz]

Details:

• Changed the type of the weights_path parameter and member variable from std::string to std::filesystem::path in the InputModel and InputModelIRImpl classes
• Updated the lambda function and return statement in FrontEnd::load_impl

TODO - type of model->get_rt_info()["__weights_path"]

Tickets:

• CVS-183679

AI Assistance:

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• If yes, summarize how AI was used and what human validation was performed (build/tests/manual checks).