3.x: Add Completable.sequenceEqual

[akarnokd]

The operator was available in all other classes.

Related #6852

[codecov]

Codecov Report

Merging #6884 into 3.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6884      +/-   ##
============================================
- Coverage     98.29%   98.28%   -0.02%     
  Complexity     6548     6548              
============================================
  Files           735      735              
  Lines         47163    47166       +3     
  Branches       6432     6432              
============================================
- Hits          46359    46356       -3     
- Misses          289      294       +5     
- Partials        515      516       +1

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <100%> (ø)	135 <1> (+1)	⬆️
...xjava3/internal/observers/FutureMultiObserver.java	86.44% <0%> (-8.48%)	24% <0%> (-2%)
...l/operators/observable/ObservableFlatMapMaybe.java	92.95% <0%> (-2.82%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.75% <0%> (-2.09%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ava3/internal/operators/parallel/ParallelJoin.java	91.66% <0%> (-1.52%)	3% <0%> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	96.44% <0%> (-1.3%)	6% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	92.63% <0%> (-1.06%)	3% <0%> (ø)
...rxjava3/internal/schedulers/ExecutorScheduler.java	95.89% <0%> (-0.92%)	10% <0%> (ø)
.../operators/observable/ObservableCombineLatest.java	96.17% <0%> (-0.64%)	6% <0%> (ø)
... and 12 more

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