Move ClientBatchCheck and Write to use conc

We introduced [conc](https://github.com/sourcegraph/conc) to handle batching in https://github.com/openfga/sdk-generator/pull/532. For consistency, `ClientBatchCheck` and `Write` should be moved to use it too.

_Originally posted by @rhamzeh in https://github.com/openfga/sdk-generator/pull/532#pullrequestreview-2744050412_
            