3.x: Add safeSubscribe to Maybe, Single & Completable

[akarnokd]

Add the final missing method, safeSubscribe to Maybe, Single & Completable.

Resolves #6852

[codecov]

Codecov Report

Merging #6887 into 3.x will decrease coverage by 0.08%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6887      +/-   ##
============================================
- Coverage     98.32%   98.24%   -0.09%     
- Complexity     6576     6598      +22     
============================================
  Files           736      739       +3     
  Lines         47235    47329      +94     
  Branches       6433     6433              
============================================
+ Hits          46446    46497      +51     
- Misses          280      308      +28     
- Partials        509      524      +15

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	210 <1> (+1)	⬆️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	188 <1> (+1)	⬆️
...a3/internal/observers/SafeCompletableObserver.java	100% <100%> (ø)	6 <6> (?)
.../rxjava3/internal/observers/SafeMaybeObserver.java	100% <100%> (ø)	8 <8> (?)
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <100%> (ø)	140 <1> (+1)	⬆️
...rxjava3/internal/observers/SafeSingleObserver.java	100% <100%> (ø)	6 <6> (?)
...l/operators/observable/ObservableFlatMapMaybe.java	83.8% <0%> (-10.57%)	2% <0%> (ø)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	89.9% <0%> (-8.26%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	90.47% <0%> (-4.77%)	2% <0%> (ø)
...xjava3/internal/observers/FutureMultiObserver.java	91.52% <0%> (-3.39%)	25% <0%> (-1%)
... and 16 more

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