Add SharePoint Loader (#4284)
- Added a loader (`SharePointLoader`) that can pull documents (`pdf`,
`docx`, `doc`) from the [SharePoint Document
Library](https://support.microsoft.com/en-us/office/what-is-a-document-library-3b5976dd-65cf-4c9e-bf5a-713c10ca2872).
- Added a Base Loader (`O365BaseLoader`) to be used for all Loaders that
use [O365](https://github.com/O365/python-o365) Package
- Code refactoring on `OneDriveLoader` to use the new `O365BaseLoader`.
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Co-authored-by: Bagatur <baskaryan@gmail.com>
