Fixes #16629: Use only one ZIO threadpool to enhance performances

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https://issues.rudder.io/issues/16629

This change leads to a 20% improvement in Rudder policy generation time, from around 25 minutes to below 20 minutes.

This change allowed to workaround three things described here: zio/zio#1275 (comment)

• 1/ switching from a thread to an other cost a bit,
• 2/ switching on the same threadpool is idempotent but all the fixe costs, like fiber pause etc, are paid (while it should be identity)
• 3/ effectBlocking manage the discarding of the underlying thread, which we generally don't want.

The last two are already corrected in ZIO-RC18! And the first one is not that bad anymore.
So, I'm not sure I we should commit that complicated PR or just wait for RC18 (which is stabilising and will likely be ZIO 1.0.0)

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We need to test ZIO-RC18 snapshot before merging that one.

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Following perf feedback from RC18-2 (#2817) we don't need that one anymore.