3.x: Add Maybe/Single/Completable blockingSubscribe

[akarnokd]

Add the missing blockingSubscribe methods to Maybe, Single and Completable for symmetry with Flowable and Observable.

Usually we'd like to avoid blocking but now that Project Loom (preview) transparently turns latch-based awaiting into suspension inside Virtual Threads, they will act as free interop operators for exiting the reactive world into the continuation world.

Related: #6852

Marbles:

Maybe

Single

Completable

[codecov]

Codecov Report

Merging #6862 into 3.x will decrease coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6862      +/-   ##
============================================
- Coverage     98.28%   98.24%   -0.04%     
- Complexity     6380     6413      +33     
============================================
  Files           715      716       +1     
  Lines         46524    46664     +140     
  Branches       6387     6396       +9     
============================================
+ Hits          45726    45845     +119     
- Misses          282      296      +14     
- Partials        516      523       +7

Impacted Files	Coverage Δ	Complexity Δ
...ava3/internal/observers/BlockingMultiObserver.java	100% <100%> (ø)	23 <4> (+4)	⬆️
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	183 <5> (+5)	⬆️
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <100%> (ø)	124 <4> (+4)	⬆️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	155 <4> (+4)	⬆️
...nal/observers/BlockingDisposableMultiObserver.java	100% <100%> (ø)	20 <20> (?)
.../operators/observable/ObservableFlatMapSingle.java	88.88% <0%> (-6.35%)	2% <0%> (ø)
...java3/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	94.87% <0%> (-3.42%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.7% <0%> (-2.61%)	2% <0%> (ø)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.55% <0%> (-2.47%)	50% <0%> (ø)
... and 21 more

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