3.x: Add timeInterval & timestamp to M/S

[akarnokd]

Add timeInterval and timestamp operators (4 overloads each). to Maybe and Single.

Related #6852

[codecov]

Codecov Report

Merging #6874 into 3.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6874      +/-   ##
============================================
- Coverage     98.27%   98.26%   -0.02%     
- Complexity     6502     6523      +21     
============================================
  Files           731      733       +2     
  Lines         46968    47044      +76     
  Branches       6420     6424       +4     
============================================
+ Hits          46159    46226      +67     
- Misses          291      295       +4     
- Partials        518      523       +5

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	170 <8> (+8)	⬆️
.../internal/operators/single/SingleTimeInterval.java	100% <100%> (ø)	2 <2> (?)
...a3/internal/operators/maybe/MaybeTimeInterval.java	100% <100%> (ø)	2 <2> (?)
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	198 <8> (+8)	⬆️
...in/java/io/reactivex/rxjava3/schedulers/Timed.java	100% <100%> (ø)	12 <1> (-1)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	83.8% <0%> (-10.57%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.09% <0%> (-5.56%)	2% <0%> (ø)
...java3/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.55% <0%> (-2.47%)	50% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
... and 25 more

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