Returns quota usage details for the asset root with the given ID.

Usage notes:

  - The id must be a root folder like "users/foo" (not "users/foo/bar").

  - The authenticated user must own the asset root to see its quota usage.

UsageReturns, callback)AssetQuotaDetails
rootIdStringThe ID of the asset root to check, e.g. "users/foo".
callbackFunction, optionalAn optional callback. If not supplied, the call is made synchronously.