Can IndexSearcher enable search concurrency by default? [LUCENE-10487]

[asfimport]

IndexSearcher has the ability to search an index using multiple threads, but this behavior is an opt-in. Could we enable it by default so that:
 - Users experience better latency out-of-the-box.
 - Lucene performs better in comparative benchmarks against other engines that leverage multiple threads by default.

Note that this isn't totally straightforward to implement, as we'd need to share the threadpool across all index searchers and think of when to shut down this threadpool?

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Migrated from LUCENE-10487 by Adrien Grand (@jpountz)

[jpountz]

Optimistically adding this issue to the 10.0 milestone.

[javanna]

This should probably be taken into consideration again. With #13472, we can now use the same executor provided to the IndexSearcher constructor, to call IndexSearcher#search. That simplifies the requirements for concurrent searches quite a bit, in that you no longer as a user need to think about sizing and providing a separate workers thread pool. Can we leverage that to enable concurrency by default?