Is it able to decouple tokio, allow to use custom async executor?

Now datafusion deeply relies on tokio, is it able to allow user to use custom async runtime? Such as `with_executor` method of Hyper: https://docs.rs/hyper/0.14.16/hyper/server/conn/struct.Http.html#method.with_executor

Start by reviewing where DataFusion relies on Tokio and compare the requested capability with Hyper's `with_executor` method. Define the executor boundary and identify the affected APIs and tests before making changes; done means users can provide a custom async runtime without the existing Tokio coupling.

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