feat(operators)!: introduce a buffer size parameter to Multi::emitOn

[jponge]

The operator was previously using a hard-coded buffer size of 16 elements which was also used to batch upstream demand.
This was a performance bottleneck for large streams as observed in the field.

The existing Multi::emitOn(executor) method remains, but it now uses Infrastructure.getBufferSizeS() as a default buffer size instead of 16.

We do not expect existing usages of emitOn(executor) to be affected by this internal change.

Fixes: #1758
See-Also: quarkusio/quarkus#36177

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.45%. Comparing base (38be323) to head (2d225a8).
Report is 20 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1761      +/-   ##
============================================
- Coverage     89.48%   89.45%   -0.03%     
+ Complexity     3287     3281       -6     
============================================
  Files           458      458              
  Lines         13365    13368       +3     
  Branches       1651     1651              
============================================
- Hits          11960    11959       -1     
- Misses          787      792       +5     
+ Partials        618      617       -1     

Files with missing lines	Coverage Δ
...tation/src/main/java/io/smallrye/mutiny/Multi.java	94.44% <ø> (ø)
...va/io/smallrye/mutiny/operators/AbstractMulti.java	100.00% <100.00%> (ø)
...smallrye/mutiny/operators/multi/MultiEmitOnOp.java	91.42% <100.00%> (-1.91%)	⬇️

... and 9 files with indirect coverage changes