3.x: Add Single.mergeArray & mergeArrayDelayError

[akarnokd]

Single was missing the mergeArray and mergeArrayDelayError operators already present in the other classes.

Related #6852

[codecov]

Codecov Report

Merging #6882 into 3.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6882      +/-   ##
============================================
- Coverage     98.29%   98.27%   -0.03%     
- Complexity     6548     6550       +2     
============================================
  Files           735      735              
  Lines         47163    47165       +2     
  Branches       6432     6432              
============================================
- Hits          46359    46351       -8     
- Misses          289      294       +5     
- Partials        515      520       +5

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <ø> (ø)	204 <0> (ø)	⬇️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	182 <2> (+2)	⬆️
...nternal/operators/observable/ObservableCreate.java	94.01% <0%> (-4.28%)	2% <0%> (ø)
...xjava3/internal/observers/FutureMultiObserver.java	91.52% <0%> (-3.39%)	25% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.7% <0%> (-3.13%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-2.11%)	3% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-1.14%)	3% <0%> (ø)
...3/internal/operators/flowable/FlowablePublish.java	97.53% <0%> (-0.99%)	16% <0%> (-1%)
...a3/internal/operators/flowable/FlowableCreate.java	97.08% <0%> (-0.65%)	6% <0%> (ø)
... and 16 more

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