impl(rest): add completion queue and thread types with no grpc deps

[scotthart]

Upcoming OAuth features require timers and background processing. The existing REST completion queue and background thread types still depend on gRPC types from the original CompletionQueue making them unsuitable. If we had known a few years ago we'd need this, it would have been done differently then.

This PR adds the needed "pure" REST versions of background threads and completion queue. moves RunAsyncBase to its own file in the common library, and implements RestCompletionQueueImpl using the new RestPureCompletionQueueImpl which is still based on TimerQueue.

Hopefully we can reconcile these various types in the future and reduce the need for all the varieties.

[gemini-code-assist]

Code Review

This pull request introduces "pure REST" implementations for background threads and completion queues to decouple the REST library from gRPC dependencies. It refactors RestCompletionQueueImpl to use these new components and moves RunAsyncBase to a dedicated header. Feedback recommends adding a joinable check before joining threads in the background thread pool and removing an unused std::enable_shared_from_this inheritance to simplify the RestPureCompletionQueueImpl class.

[codecov]

Codecov Report

❌ Patch coverage is 36.36364% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.67%. Comparing base (efaacf4) to head (13e8938).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...loud/internal/rest_pure_background_threads_impl.cc	0.00%	30 Missing ⚠️
...e/cloud/internal/rest_pure_completion_queue_impl.h	42.85%	4 Missing ⚠️
.../cloud/internal/rest_pure_completion_queue_impl.cc	82.35%	3 Missing ⚠️
google/cloud/internal/rest_completion_queue_impl.h	33.33%	2 Missing ⚠️
...cloud/internal/rest_pure_background_threads_impl.h	0.00%	2 Missing ⚠️
...oogle/cloud/internal/rest_completion_queue_impl.cc	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16078      +/-   ##
==========================================
- Coverage   92.69%   92.67%   -0.02%     
==========================================
  Files        2343     2348       +5     
  Lines      217439   217492      +53     
==========================================
+ Hits       201547   201555       +8     
- Misses      15892    15937      +45     

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[jinseopkim0]

Would it make sense to add unit tests?

[scotthart]

The implementation for PureRestCompletionQueue* was ported over from RestCompletionQueue* and then RestCompletionQueue* is implemented using PureRestCompletionQueue*. So, the existing unit tests for RestCompletionQueue* are testing PureRestCompletionQueue*.