Feat: WithAsync has no meaning if Async insert is enabled by default on server

1. ClickHouse made async insert as [default in recent version](https://clickhouse.com/docs/operations/settings/settings#async_insert)
2. Meaning, even without setting `async_insert=1` explicitly (which is basically what `WithAsync` option do) user will have async_insert on the server side.

We need some way to disable async_insert if user wants to. Also gives better user experience

Start by tracing the Go driver's WithAsync option and its handling of the async_insert setting. Inspect any existing async-insert tests or entry points to understand current behavior. Done means users have a clear way to disable async_insert when the ClickHouse server enables it by default, with tests covering that choice.

Written by the indexing model from the issue text.