This is done by making fs.Config private and attaching it to the
context instead.
The Config should be obtained with fs.GetConfig and fs.AddConfig
should be used to get a new mutable config that can be changed.
This enables the use of the SharePoint webdav endpoint provided by
OneDrive for Business or Office365 Education Accounts. It enables
unverified accounts to be accessed with rclone via webdav as it isn't
possible through the normal onedrive backend.
This integrates the https://github.com/hensur/onedrive-cookie-test
package to fetch the required cookies to authorize against the
SharePoint webdav endpoint.
