documents: Add Shared Drives support
It seems that the existing documents APIs works nicely with the files on Shared Drives (formerly Team Drives), although it is possible that some small changes will be needed to handle differences: https://developers.google.com/drive/api/v2/shared-drives-diffs
However, the main problem is that there is no way currently to get the list of Shared Drives IDs and their names. Let's add new APIs for it which will be consequently used by GVfs.
Relates: gvfs#377 (closed)
Relates: gvfs!114 (merged)
Fixes: #25 (closed)