3.x: Add onDropped callback to throttleLatest operator

[akarnokd]

This PR adds an overload to the throttleLatest operator which takes an onDropped consumer.

This consumer is called when the latest known item is replaced by a newer item, or the sequence gets terminated with a latest known item. It should allow some per-item cleanup to happen.

If the onDropped crashes, the exception is either delivered to the downstream (if the sequence is still live at that point) or to the global error handler (sequence is stopped).

(Also fixed a bug in one of the test helpers regarding asserting composite exception members.)

[codecov]

Codecov Report

Merging #7457 (f54fafa) into 3.x (ef51a90) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7457      +/-   ##
============================================
+ Coverage     99.55%   99.56%   +0.01%     
- Complexity     6781     6784       +3     
============================================
  Files           751      751              
  Lines         47482    47579      +97     
  Branches       6378     6393      +15     
============================================
+ Hits          47271    47373     +102     
  Misses           93       93              
+ Partials        118      113       -5     

Impacted Files	Coverage Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <100.00%> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <100.00%> (ø)
...nal/operators/flowable/FlowableThrottleLatest.java	100.00% <100.00%> (ø)
...operators/observable/ObservableThrottleLatest.java	100.00% <100.00%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	91.54% <0.00%> (-4.93%)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	⬇️
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	99.16% <0.00%> (-0.42%)	⬇️
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.34% <0.00%> (ø)
...nternal/operators/observable/ObservableWindow.java	100.00% <0.00%> (+1.78%)	⬆️
... and 5 more

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