[FEATURE REQUEST] Python Bindings: Support for Pickling

It seems that pickling support is required for easily using ImageBuf / ImageSpec objects in multiprocessing in python. Therefore it would be great if pickling support could be added to them along these lines:
https://pybind11.readthedocs.io/en/stable/advanced/classes.html#pickling-support

For meta data in ImageSpec, using to_xml() from_xml() could be an alternative although it does seem that from_xml() is not fully implemented to capture all meta data. There may be other alternatives which I am not aware of.

Thanks for consideration.

Start with the Python bindings for ImageBuf and ImageSpec and the pybind11 pickling-support guidance linked in the issue. Compare the proposed to_xml()/from_xml() metadata route, noting that from_xml() may not preserve all metadata. Done means both objects can be pickled for multiprocessing with the required ImageSpec metadata retained.

Written by the indexing model from the issue text.