Support returning sync token values after committing transactions (batching).

[AhmedZaki99]

As stated in the official documentation under Add two new tasks section:

In the results, we see the following data:

• A new sync_token which we can use later on to do more incremental syncs.

These sync tokens returned by batching requests can be very helpful in keeping the sync state.

My suggestion here is to make the ITransaction.CommitAsync() return Task<string> that when completed returns the sync_token value, instead of just empty Task.

That change is expected not to have side effects since the Task<string> behaves exactly like a plain Task but with a result.

Thanks in advance for your time and for the great package 🙂

[AhmedZaki99]

I've opened a pull request for the suggested changes.