feat: flag to control the Infrastructure executor shutdown on update

[jponge]

There are cases where we might not want to shut the previous executor down, so an extra flag and update method allow more fine-grained framework integrations.

[jponge]

This can be useful for quarkusio/quarkus#50918

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.16%. Comparing base (e728220) to head (9690251).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
...smallrye/mutiny/infrastructure/Infrastructure.java	75.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2020      +/-   ##
============================================
+ Coverage     89.01%   89.16%   +0.14%     
- Complexity     3063     3107      +44     
============================================
  Files           409      412       +3     
  Lines         13105    13266     +161     
  Branches       1654     1682      +28     
============================================
+ Hits          11666    11828     +162     
+ Misses          814      810       -4     
- Partials        625      628       +3     

Files with missing lines	Coverage Δ
...smallrye/mutiny/infrastructure/Infrastructure.java	72.09% <75.00%> (+0.05%)	⬆️

... and 13 files with indirect coverage changes

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