feat: support union and optional arguments

[jpbrodrick89]

Description of changes

Support union and optional types (e.g. float | str, str | None, Hobby | list[Hobby]:

Resolution rules:
1. If null is in union, remove it and set is_optional=True
2. If any member has $ref, resolve to "json"
3. If only int/float types remain, resolve to "number"
4. If array + only int/float, resolve to "array"
5. If has number + other non-composite types, resolve to "string" with could_be_number=True
6. Otherwise resolve to "string" with could_be_number=False

could_be_number means that text input renders 42 as a number (Pydantic's smart resolution would resolve "42" as a string).

Added checkboxes for optional arguments if unselected then None is passed. Unfortunately, removing the input field conditionally is not possible in a jinja template. Any reason why we're using jinja templates and not pure streamlit here?

Testing done

⚠️ Made some changes to test_app that weren't directly related to this PR, I removed the output field dummy which is not in the OutputSchema and changed int.hobby.name to str.hobby.name. I have no idea how tests passed previously...

Added test cases to mock schemas and goodbyeworld.
Added unit tests for parsing functions.
Manual testing.

[dionhaefner]

Looks really useful, thanks. @jacanchaplais are you taking this one?

[jacanchaplais]

Thanks both @dionhaefner and @jpbrodrick89. Yep, will review this today or tomorrow. :)