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ARROW-13806: [C++][Python] Add support for new MonthDayNano Interval Type

- Refactored ObjectWriter helpers from arrow_to_pandas, so they can be
  used for plain python types as well (generalized the lowest level so
  it can work on both PyObject** and an adapter for PyList.

- Add DateOffset to static pandas imports

- Tried to start laying out code in a way to use C++ for Array.to_pylist
  (feel free to comment).

Support importing from timeinterval, relativedelta and DateOffset types
(this is actually mostly duck types, the one complication is that
 relativedelta has a property weeks that is automatically calculated, so
 some type checking is necessary).

Open questions:
 - Should we be more strict on duck typing imports? I chose generalism
   over performance here (rechecking non-present attributes, etc)?
 - Is the new arrow_to_python.h desirable (I think this can be easily
   extended for other types)?
 - My python is rusty and Python C-API even more so, please don't assume
   I know exactly what I'm doing :)

Closes #11302 from emkornfield/interval_python

Lead-authored-by: Micah Kornfield <micahk@google.com>
Co-authored-by: emkornfield <emkornfield@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>