Upload additional files in run object

[Taniya-Das]

Description

Implement upload of additional files (such as model in binary format etc.) in the run object.

Steps

To upload additional files, each file should be added as a tuple ("file_name", "file_content") to file_elements in https://github.com/openml/openml-python/blob/develop/openml/base.py#L132.

This has been implemented as a patch in this commit - openml/openml-tensorflow@259f356

Example XML with additional file

Example: https://www.openml.org/api/v1/xml/run/10594197

[PGijsbers]

Specifically there is already a method for extensions that is supposed to be able to let the extension add additional files to the run, but it is not used from the generic extension code (supposedly). But it would also be good if the run object had some add_file function or something of that nature to allow anyone to add the extra files, and update OpenMLRun._get_file_elements accordingly.