Merge to 1.1.x: Make Pub/Sub thread pools Spring-managed beans (#1505)

[elefeint]

The two thread pools are hardcoded in autoconfiguration, so they fall through the cracks when a Spring application shuts down:

• Spring does not manage them because they are not context-managed beans.
• gRPC stub does not manage them because FixedExecutorProvider.shouldAutoClose() is false (also, we don't call SubscriberStub.shutdown(), which we arguably should do, as well.

This does not manifest on the publisher side because that thread pool is never started, but the subscriber thread pool is started by way of subscriber stub creation when PubSubSubscriberTemplate is instantiated.

Cherrypicked from #1505.
Fixes #1503.
Addresses the Pub/Sub part of #1506.

[codecov-io]

Codecov Report

Merging #1518 into 1.1.x will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##              1.1.x    #1518      +/-   ##
============================================
+ Coverage     68.29%   68.33%   +0.03%     
- Complexity     1429     1431       +2     
============================================
  Files           203      203              
  Lines          5728     5735       +7     
  Branches        578      578              
============================================
+ Hits           3912     3919       +7     
  Misses         1579     1579              
  Partials        237      237

Flag	Coverage Δ	Complexity Δ
#integration	?	?
#unittests	68.33% <100%> (+0.03%)	1431 <4> (+2)	⬆️

Impacted Files	Coverage Δ	Complexity Δ
...toconfigure/pubsub/GcpPubSubAutoConfiguration.java	90.47% <100%> (+0.56%)	38 <4> (+2)	⬆️

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