Returns permissions that the caller has on the specified database or backup resource. Attempting this RPC on a non-existent Cloud Spanner database will result in a NOT_FOUND error if the user has spanner.databases.list permission on the containing Cloud Spanner instance. Otherwise returns an empty set of permissions. Calling this method on a backup that does not exist will result in a NOT_FOUND error if the user has spanner.backups.list permission on the containing instance. Calling this method on a backup schedule that does not exist will result in a NOT_FOUND error if the user has spanner.backupSchedules.list permission on the containing database.