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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