3.x: Add NonNull & SafeVarargs annotations + validator

[akarnokd]

This PR cleans up the main classes:

• Add missing @NonNull annotations
• Add missing @SafeVarargs annotations
• Add validator code that scans the sources to verify the annotations are present
• Remove unnecessary @SuppressWarnings annotations
• Fix a few type arguments.

Related #6766

[codecov]

Codecov Report

Merging #6791 into 3.x will decrease coverage by 0.07%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##                3.x   #6791      +/-   ##
===========================================
- Coverage     98.27%   98.2%   -0.08%     
+ Complexity     6287    6286       -1     
===========================================
  Files           696     696              
  Lines         45390   45395       +5     
  Branches       6233    6233              
===========================================
- Hits          44607   44579      -28     
- Misses          273     290      +17     
- Partials        510     526      +16

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	577 <0> (ø)	⬇️
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <100%> (ø)	537 <30> (ø)	⬇️
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	177 <18> (ø)	⬇️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	151 <10> (ø)	⬇️
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <100%> (ø)	120 <4> (ø)	⬇️
...n/java/io/reactivex/rxjava3/core/Notification.java	93.93% <100%> (ø)	22 <1> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	83.8% <0%> (-11.27%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	92.85% <0%> (-3.18%)	2% <0%> (ø)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.58% <0%> (-2.44%)	51% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-2.11%)	3% <0%> (ø)
... and 18 more

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