3.x: Move nullability annotations into the type argument declaration

[akarnokd]

This PR makes sure generic type argument declarations on methods include the @NonNull (or @Nullable) annotation as well as source-checking test code to verify the expected style.

Before:

public final <R> Maybe<R> map(@NonNull Function<? super T, ? extends R> mapper) {
// or
public final <R> Maybe<R> map(@NonNull Function<? super T, @NonNull ? extends R> mapper) {

After:

public final <@NonNull R> Maybe<R> map(@NonNull Function<? super T, ? extends R> mapper) {

Related #7302

[codecov]

Codecov Report

Merging #7303 (14831e6) into 3.x (bb814d2) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7303      +/-   ##
============================================
- Coverage     99.53%   99.50%   -0.03%     
+ Complexity     6783     6781       -2     
============================================
  Files           751      751              
  Lines         47489    47489              
  Branches       6378     6378              
============================================
- Hits          47266    47253      -13     
- Misses          102      110       +8     
- Partials        121      126       +5     

Impacted Files	Coverage Δ
...in/java/io/reactivex/rxjava3/core/Completable.java	100.00% <ø> (ø)
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <ø> (ø)
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100.00% <ø> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <ø> (ø)
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100.00% <ø> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	100.00% <ø> (ø)
...o/reactivex/rxjava3/parallel/ParallelFlowable.java	100.00% <ø> (ø)
...o/reactivex/rxjava3/processors/AsyncProcessor.java	97.91% <ø> (ø)
.../reactivex/rxjava3/subscribers/SafeSubscriber.java	100.00% <ø> (ø)
...ernal/subscriptions/BasicIntQueueSubscription.java	100.00% <100.00%> (ø)
... and 24 more

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[ZacSweers]

This is great!! Will be a big win for nullaway users and kotlin users starting in 1.5.30.