Added handling of filename_as_id and file_extractor to SharePointReader

[ferdinandosimonetti]

Description

I've taken MinioReader's handling of file_extractor parameter for SimpleDirectoryReader This allows to choose a customized matching between file extension and its Reader/Decoder, and *shouldn't wreak havoc on SharePointReader's functionality.

Type of Change

Please delete options that are not relevant.

• [X] Bug fix / Smaller change

How Has This Been Tested?

• [X] I stared at the code and made sure it makes sense

Suggested Checklist:

• [ ] I have added a library.json file if a new loader/tool was added
• [X] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] I ran make format; make lint to appease the lint gods