feat(internal): thread pool

[jpnurmi]

Introduce a generic thread pool that runs tasks in parallel in a predetermined number of pooled threads and invokes completion callbacks in submission order. The primary use case is the telemetry serialization pool coming in #1946, but it could be used for offloading other heavy tasks, too.

See usage in #1946:

• Pool creation:
  

  sentry-native/src/sentry_telemetry.c

  Lines 27 to 28 in 1134ee2

  	// use two workers for serializing telemetry batches off the batcher threads
  	g_telemetry_pool = sentry__threadpool_new(2);

• Batch submission:
  

  sentry-native/src/sentry_batcher.c

  Lines 477 to 478 in 1134ee2

  	if (sentry__threadpool_submit(batcher->threadpool, batch_task_exec,
  	batch_task_complete_and_cleanup, NULL, task)

#skip-changelog (internal)

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[codecov]

Codecov Report

❌ Patch coverage is 84.46970% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.31%. Comparing base (219d934) to head (7f9c7b5).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1883      +/-   ##
==========================================
- Coverage   76.01%   74.31%   -1.71%     
==========================================
  Files          94      104      +10     
  Lines       22183    25625    +3442     
  Branches     3937     4622     +685     
==========================================
+ Hits        16863    19043    +2180     
- Misses       4436     5283     +847     
- Partials      884     1299     +415     

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[limbonaut]

Epic work!🦸 The chart from the stacked follow-up shows exactly how useful this can be. I really like the idea of a flexible ordered worker queue with pooled threads. Quite a brain-crack to review 😄 I only have one concern, and a naming nit.

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[JoshuaMoelans]

This is a very cool feature, learned a lot reading up on thread-pooling! LGTM, with just a nit and two questions remaining.