2.x: add subscribeOn overload to avoid same-pool deadlock with create

[akarnokd]

This PR adds an overload to subscribeOn that exposes the existing feature to optionally request on the same scheduler where the subscription happened. This is necessary to avoid same-pool deadlock when the upstream contains create logic that blocks the emission thread, preventing any scheduled request to get through and leading to excess buffering or dropping data excessively. By not scheduling the request, it can directly update the emitter's request tracking and let the emitter continue.

Formerly, the existing subscribeOn automatically disabled scheduling the requests if the immediate upstream was a FlowableCreate. However, if there were operators between create and subscribeOn (as often happening on Android with composing the schedulers at the end of the chain), the subscribeOn operator run in scheduled request mode by default.

This change allows specifying this behavior through the new overload and thus distance between the create and subscribeOn operators no longer matters.

(Note that 1.x already has this overload.)

[codecov]

Codecov Report

Merging #5386 into 2.x will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##               2.x    #5386      +/-   ##
===========================================
+ Coverage     96.1%   96.13%   +0.03%     
- Complexity    5781     5786       +5     
===========================================
  Files          630      630              
  Lines        41195    41197       +2     
  Branches      5726     5728       +2     
===========================================
+ Hits         39591    39606      +15     
+ Misses         632      626       -6     
+ Partials       972      965       -7

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/flowable/FlowableSubscribeOn.java	91.52% <100%> (ø)	2 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	523 <2> (+2)	⬆️
...al/operators/observable/ObservableSampleTimed.java	88.33% <0%> (-10%)	3% <0%> (ø)
...rnal/subscriptions/ArrayCompositeSubscription.java	93.33% <0%> (-6.67%)	17% <0%> (-1%)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <0%> (-4.62%)	27% <0%> (-1%)
...rnal/operators/completable/CompletableTimeout.java	94% <0%> (-4%)	2% <0%> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...vex/internal/operators/single/SingleTakeUntil.java	91.8% <0%> (-3.28%)	2% <0%> (ø)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
... and 43 more

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