3.x: Add Maybe/Completable toFuture

[akarnokd]

Add toFuture to Maybe and Completable.

Related #6852

[codecov]

Codecov Report

Merging #6875 into 3.x will increase coverage by 0.02%.
The diff coverage is 77.77%.

@@             Coverage Diff              @@
##                3.x    #6875      +/-   ##
============================================
+ Coverage     98.24%   98.27%   +0.02%     
- Complexity     6523     6527       +4     
============================================
  Files           733      733              
  Lines         47044    47052       +8     
  Branches       6424     6425       +1     
============================================
+ Hits          46217    46238      +21     
+ Misses          303      294       -9     
+ Partials        524      520       -4

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	588 <0> (ø)	⬇️
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <ø> (ø)	558 <0> (ø)	⬇️
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	199 <1> (+1)	⬆️
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <100%> (ø)	129 <1> (+1)	⬆️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	170 <1> (ø)	⬇️
...xjava3/internal/observers/FutureMultiObserver.java	94.91% <66.66%> (ø)	26 <1> (?)
.../operators/observable/ObservableFlatMapSingle.java	88.09% <0%> (-5.56%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...java3/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	95% <0%> (-1.67%)	2% <0%> (ø)
... and 18 more

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