feat(kotlin): add switchIfEmpty with deferred execution

[davinkevin]

This allows supporting a deferred execution when using the switchIfEmpty operator

[codecov-io]

Codecov Report

Merging #1460 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #1460      +/-   ##
============================================
- Coverage     84.29%   84.25%   -0.04%     
+ Complexity     3894     3893       -1     
============================================
  Files           358      358              
  Lines         29426    29427       +1     
  Branches       5464     5464              
============================================
- Hits          24804    24794      -10     
- Misses         3026     3031       +5     
- Partials       1596     1602       +6

Impacted Files	Coverage Δ	Complexity Δ
...in/kotlin/reactor/core/publisher/MonoExtensions.kt	83.33% <ø> (ø)	0 <0> (ø)	⬇️
...in/kotlin/reactor/core/publisher/FluxExtensions.kt	88.46% <0%> (-3.54%)	0 <0> (ø)
.../java/reactor/core/publisher/FluxLimitRequest.java	94% <0%> (-4%)	5% <0%> (ø)
.../java/reactor/core/scheduler/ElasticScheduler.java	83.47% <0%> (-1.66%)	26% <0%> (ø)
...in/java/reactor/core/publisher/FluxWindowWhen.java	80.28% <0%> (-1.45%)	2% <0%> (ø)
...c/main/java/reactor/core/publisher/RingBuffer.java	70.56% <0%> (-0.95%)	24% <0%> (ø)
.../main/java/reactor/core/publisher/FluxFlatMap.java	94.4% <0%> (-0.58%)	13% <0%> (ø)
...ain/java/reactor/core/publisher/FluxConcatMap.java	89.85% <0%> (-0.57%)	7% <0%> (ø)
...c/main/java/reactor/core/publisher/FluxReplay.java	83.85% <0%> (-0.32%)	24% <0%> (-1%)
...ain/java/reactor/core/publisher/FluxPublishOn.java	86.59% <0%> (-0.21%)	6% <0%> (ø)
... and 6 more

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[simonbasle]

Looks good to me, can you change the @since TBD to @since 3.2?

[davinkevin]

Modification made. I've set version 3.2 on the @since.

I've also modified a signature method to accept Publisher instead of Mono in the FluxExtensions.kt.

👍

[simonbasle]

"a function providing a Mono" -> "a function providing a Publisher"