Sync#fetch should return the number of items remaining in queue

[fightingtheboss]

It would be tremendously helpful to be able to query the number of items remaining in the queue when using the Sync API. The API endpoint for sync does deliver this info, but it's ignored in this wrapper gem. Having this info available is particularly useful when doing an initial sync with Base, when there could be thousands of records and this could take quite a long time.

I propose that an attr_reader is added to the SyncService object (e.g. count_left as it's called in the API response) such that it can be queried in the body of the .fetch method block.