CORE-5321 Updated share endpoint to use permission keyword.

Added a set-permission function to clj-jargon.permissions.
Updated donkey.services.sharing to parse a 'permission' keyword from the request JSON instead of a 'permissions' object.
Updated donkey.services.filesystem.sharing/share and related functions to use the new set-permission function with the 'permission' keyword from the request instead of a 'permissions' object.
Updated sharing docs for new request JSON format.