Support for custom Pydantic fields

[an23rd]

Thanks for making this repo @lukasmasuch!
I would love to use your library in my repo but I would need to make it work with custom Pydantic fields like your FileContent class. How would you integrate that feature? I see that the elements are rendered by the various methods like def _render_single_file_input of InputUI. I think it would be really nice if it was possible to add custom renderers for specific classes. Maybe kept in a dictionary. Then every user can add their elements if desired and at rendering the dictionary is scanned first for a matching entry. What do you think?