Sometimes it is acceptable to users to only search indexed data and skip
and new un-indexed data. For example, if un-indexed data will be shortly
indexed and they don't mind the delay. In these cases, we can save a lot
of CPU time in search, and provide better latency. Users can activate
this on queries using `fast_search()`.
It's useful to see the underlying query plan for debugging purposes.
This exposes LanceScanner's `explain_plan` function. Addresses #1288
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Co-authored-by: Will Jones <willjones127@gmail.com>