Allow creating additive nested folder shares

[shepilov]

Summary

Once AccessResolver.Resolve(target) exists, update shared-drive creation so a folder inside an existing shared folder can be shared with additional users.

TODO

Update shared-drive root validation for additive nested folder sharing:

• Keep rejecting the selected root if it is already directly shared.
• Stop rejecting the selected root only because an ancestor is shared.
• Stop rejecting the selected root only because a descendant is shared.

On POST /sharings/drives:

• resolve effective access on the selected folder
• require write/share permission in the effective parent scope
• create the child sharing with access_mode = additive
• add referenced_by to the child folder root
• do not copy inherited parent members into the child sharing

Out Of Scope

• Move/copy
• Delete/trash
• Realtime/sync
• Limited access
• UI inherited/direct member display