Add types for `DataFrame.itertuples()` and `DataFrame.iterrows()`

Right now, `DataFrame.itertuples()` and `DataFrame.iterrows()` both don't have return types.

`itertuples()` can return `Iterable[tuple[Any, ...]]` in the stubs

`iterrows()` can return `Iterable[tuple[Hashable, Series]]:` in the stubs
