3.x: Add Single.ofType

[akarnokd]

Add ofType to Single already available to the other non-empty base types.

Related #6852

In addition, diagrams for Maybe.filter and Maybe.ofType have been updated:

Related: #5806

[codecov]

Codecov Report

Merging #6876 into 3.x will increase coverage by 0.07%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## 3.x #6876 +/- ## =========================================== + Coverage 98.22% 98.3% +0.07%  - Complexity 6525 6527 +2  =========================================== Files 733 733 Lines 47052 47054 +2 Branches 6425 6425 =========================================== + Hits 46216 46255 +39  + Misses 305 289 -16  + Partials 531 510 -21

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <ø> (ø)	199 <0> (ø)	⬇️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	171 <1> (+1)	⬆️
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ternal/operators/completable/CompletableMerge.java	95.94% <0%> (-1.36%)	2% <0%> (ø)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-1.14%)	3% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.09% <0%> (-0.91%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	95.9% <0%> (-0.82%)	2% <0%> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	96.76% <0%> (-0.65%)	6% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	96.25% <0%> (-0.54%)	5% <0%> (ø)
...3/internal/operators/flowable/FlowableGroupBy.java	96.22% <0%> (+0.31%)	3% <0%> (ø)	⬇️
... and 13 more

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