feat: parse optional flat enums

[4dawit]

Goal

Closes #461

Checklist

• I have followed the steps listed in the Contributing guide.
• If necessary, I have added documentation related to the changes made.
• I have added or updated the tests related to the changes made.

[4dawit]

I don't love is_enum and optional_inline_flattened, but I couldn't figure out another way to pass in the context otherwise (couldn't access IS_OPTION with field_ty like in optional.rs since enum isn't a field, also struggled with not panicking inline_flattened() on Option<T> if T is an enum). There might be other cases to be handled (serde un/tagged, etc), please feel free to give feedback or refactor.

[4dawit]

Follow up: I rebased and see that is_enum was created for something else, any updates on implementing something or adopting changes from this PR to close #461?