3.x: Add concatEagerDelayError across

[akarnokd]

Flowable, Observable, Maybe and Single now have a concatEagerDelayError, delegating to the appropriate concatMapEagerDelayError operator.

In addition, Maybe and Single received an overload concatEager(Iterable, int) to expose the maxConcurrency option already present in the rest of the types.

Note that Completable.concatEagerDelayError is essentially the same as mergeDelayError because there are no items to keep emitting in order.

Resolves #6880

The Flowable and Observable variant of concatEager also received fresh marble diagrams. (Related #5813)

Flowable

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Observable

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Maybe

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Single

[codecov]

Codecov Report

Merging #6899 into 3.x will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6899      +/-   ##
============================================
+ Coverage     98.26%   98.26%   +<.01%     
- Complexity     6597     6618      +21     
============================================
  Files           742      742              
  Lines         47400    47426      +26     
  Branches       6443     6443              
============================================
+ Hits          46578    46604      +26     
  Misses          300      300              
  Partials        522      522

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <ø> (ø)	140 <0> (ø)	⬇️
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <100%> (ø)	565 <6> (+4)	⬆️
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	214 <7> (+6)	⬆️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	196 <8> (+6)	⬆️
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <100%> (ø)	595 <5> (+4)	⬆️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.66% <0%> (-4.69%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	91.26% <0%> (-2.39%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	92.7% <0%> (-2.19%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	95% <0%> (-1.67%)	2% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.09% <0%> (-0.91%)	2% <0%> (ø)
... and 21 more

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