3.x: [Java 8] Add flattenStreamAsX to Maybe/Single

[akarnokd]

Add

• Maybe.flattenStreamAsFlowable
• Maybe.flattenStreamAsObservable
• Single.flattenStreamAsFlowable
• Single.flattenStreamAsObservable

Related #6776

(In addition, adjust the validators to appreciate the new patterns.)

Marbles:

[codecov]

Codecov Report

Merging #6805 into 3.x will decrease coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6805      +/-   ##
============================================
- Coverage     98.27%   98.22%   -0.06%     
- Complexity     6372     6381       +9     
============================================
  Files           711      715       +4     
  Lines         46243    46496     +253     
  Branches       6329     6371      +42     
============================================
+ Hits          45446    45670     +224     
- Misses          282      299      +17     
- Partials        515      527      +12

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <ø> (ø)	553 <0> (ø)	⬇️
...a3/internal/jdk8/MaybeFlattenStreamAsFlowable.java	100% <100%> (ø)	2 <2> (?)
.../internal/jdk8/MaybeFlattenStreamAsObservable.java	100% <100%> (ø)	2 <2> (?)
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	179 <2> (+2)	⬆️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	153 <2> (+2)	⬆️
...3/internal/jdk8/SingleFlattenStreamAsFlowable.java	100% <100%> (ø)	2 <2> (?)
...internal/jdk8/SingleFlattenStreamAsObservable.java	100% <100%> (ø)	2 <2> (?)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	89.9% <0%> (-8.26%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.88% <0%> (-5.56%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
... and 18 more

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