3.x: [Java 8] Add fromOpt/Stage, mapOptional, toCompletionStage to M/S/C

[akarnokd]

Add the following Java 8 operators to various reactive base classes:

Operator	Maybe	Single	Completable
fromOptional		(1)	(2)
fromCompletionStage
mapOptional			(3)
toCompletionStage			(4)
toCompletionStage(T)		(5)

1. Should it be implemented as Optional.empty() -> Single.error(new NoSuchElementException())?
2. No reason to implement. Always Completable.complete().
3. No value to map.
4. No value to emit thus it is better to ask the user for a completion value.
5. Never empty, no reason to implement.

Related #6776

Marbles:

[codecov]

Codecov Report

Merging #6783 into 3.x will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6783      +/-   ##
============================================
+ Coverage     98.24%   98.25%   +<.01%     
- Complexity     6255     6285      +30     
============================================
  Files           690      696       +6     
  Lines         45218    45390     +172     
  Branches       6220     6233      +13     
============================================
+ Hits          44426    44598     +172     
+ Misses          276      273       -3     
- Partials        516      519       +3

Impacted Files	Coverage Δ	Complexity Δ
...java3/internal/jdk8/SingleFromCompletionStage.java	100% <100%> (ø)	2 <2> (?)
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	177 <5> (+5)	⬆️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	151 <3> (+3)	⬆️
...rxjava3/internal/jdk8/CompletionStageConsumer.java	100% <100%> (ø)	12 <12> (?)
...tivex/rxjava3/internal/jdk8/SingleMapOptional.java	100% <100%> (ø)	2 <2> (?)
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <100%> (ø)	120 <2> (+2)	⬆️
...xjava3/internal/jdk8/MaybeFromCompletionStage.java	100% <100%> (ø)	2 <2> (?)
...ctivex/rxjava3/internal/jdk8/MaybeMapOptional.java	100% <100%> (ø)	2 <2> (?)
.../internal/jdk8/CompletableFromCompletionStage.java	100% <100%> (ø)	2 <2> (?)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
... and 31 more

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