Decouple datafusion's logical optimization/conversion

[jurplel]

Remove:

        let batches = df.collect().await?;

from datafusion-optd-cli/src/exec.rs

because it will internally run datafusion's logical optimizer. We should try to call the other collect function instead, after running our own optimizer and converting the optd plan into datafusion's (impl ExecutionPlan). This will allow us to run our end-to-end optimized query on datafusion.