Implement mathematical operators

[cescoffier]

Provides:

• count - emits the number of items emitted by the upstream
• index - emits Tuple2<Long, T> for each item from the upstream. The first element of the tuple is the index (0-based), and the second if the item
• min / max - emits the min/max item from the upstream, using Java comparator
• top(x) - emits the top x items from the upstream, a new ranking is emitted every time it changes.
• sum - emits the sum of all the items emitted by the upstream
• average - emits the average of all the items emitted by the upstream

[codecov]

Codecov Report

Merging #553 (a94f1a8) into main (604c650) will increase coverage by 0.40%.
The diff coverage is 93.46%.

@@             Coverage Diff              @@
##               main     #553      +/-   ##
============================================
+ Coverage     89.73%   90.14%   +0.40%     
- Complexity     2913     3001      +88     
============================================
  Files           380      392      +12     
  Lines         11513    11666     +153     
  Branches       1450     1461      +11     
============================================
+ Hits          10331    10516     +185     
+ Misses          605      587      -18     
+ Partials        577      563      -14     

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/io/smallrye/mutiny/math/Statistic.java	72.72% <72.72%> (ø)	12.00 <12.00> (?)
...th/src/main/java/io/smallrye/mutiny/math/Math.java	90.90% <90.90%> (ø)	10.00 <10.00> (?)
.../java/io/smallrye/mutiny/math/AverageOperator.java	100.00% <100.00%> (ø)	3.00 <3.00> (?)
...in/java/io/smallrye/mutiny/math/CountOperator.java	100.00% <100.00%> (ø)	3.00 <3.00> (?)
...in/java/io/smallrye/mutiny/math/IndexOperator.java	100.00% <100.00%> (ø)	3.00 <3.00> (?)
...main/java/io/smallrye/mutiny/math/MaxOperator.java	100.00% <100.00%> (ø)	5.00 <5.00> (?)
...n/java/io/smallrye/mutiny/math/MedianOperator.java	100.00% <100.00%> (ø)	7.00 <7.00> (?)
...main/java/io/smallrye/mutiny/math/MinOperator.java	100.00% <100.00%> (ø)	5.00 <5.00> (?)
...va/io/smallrye/mutiny/math/OccurrenceOperator.java	100.00% <100.00%> (ø)	4.00 <4.00> (?)
...va/io/smallrye/mutiny/math/StatisticsOperator.java	100.00% <100.00%> (ø)	9.00 <9.00> (?)
... and 29 more

[cescoffier]

I have added:

• median - be aware that you need to store the item, so don't use it on unbounded streams
• statistics - computes the count, average, min, max, variance, standard deviation, skewness, and kurtosis (thanks to @Ladicek link)