Fix `python-fastapi` free-form objects mapping and forward ref type hints

Created by: chludwig-haufe

This PR implements the proposed fixes for #9720 (closed) and #9721 (closed):

• Free-form object model attributes are mapped to the type hint Dict[str, Any].
• To support forward type references in the type hints generated for (recursive) model classes, add from __future__ import annotations to the model template. (This is supported in Python >= 3.7.)

With these changes in place, the generated FastAPI service could actually be started.

Python technical committee: @taxpon, @frol, @mbohlool, @cbornet, @kenjones-cisco, @tomplus, @Jyhess, @arun-nalla, @spacether

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