3.x: Add X.fromSupplier()

[akarnokd]

This PR adds the fromSupplier operator to support the new Supplier interface.

The marble diagrams will be updated in a separate PR.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (3.x@edb1f71). Click here to learn what that means.
The diff coverage is 96.93%.

@@          Coverage Diff           @@
##             3.x    #6529   +/-   ##
======================================
  Coverage       ?   98.24%           
  Complexity     ?     6189           
======================================
  Files          ?      680           
  Lines          ?    44883           
  Branches       ?     6193           
======================================
  Hits           ?    44095           
  Misses         ?      253           
  Partials       ?      535

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFromCallable.java	100% <ø> (ø)	5 <0> (?)
...ernal/operators/flowable/FlowableFromCallable.java	100% <ø> (ø)	4 <0> (?)
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	566 <1> (?)
.../internal/operators/single/SingleFromSupplier.java	100% <100%> (ø)	5 <5> (?)
src/main/java/io/reactivex/Completable.java	100% <100%> (ø)	118 <1> (?)
src/main/java/io/reactivex/Observable.java	100% <100%> (ø)	541 <1> (?)
src/main/java/io/reactivex/Single.java	100% <100%> (ø)	148 <1> (?)
...operators/completable/CompletableFromSupplier.java	100% <100%> (ø)	4 <4> (?)
...ernal/operators/flowable/FlowableFromSupplier.java	100% <100%> (ø)	4 <4> (?)
...l/operators/observable/ObservableFromSupplier.java	100% <100%> (ø)	5 <5> (?)
... and 2 more

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