Make UnitOfWork not implement IDisposable

[mvarendorff2]

Having the UnitOfWork implement IDisposable caused many issues, leaks and odd multi-threading chaos. We thus make returning the UnitOfWork to the pool explicit which should not cause too much implementation overhead in the vast majority of use-cases. Calling unitOfWork.Return() will be required now however after using serviceProvider.GetSystemUserUnitOfWork() or serviceProvider.GetUserUnitOfWork(userId).

After this is merged, we can also have more confidence in upgrading to HC@14 in #67 (+ required changes).

[codecov]

Codecov Report

Attention: Patch coverage is 81.96721% with 11 lines in your changes missing coverage. Please review.

Project coverage is 86.30%. Comparing base (eb68f68) to head (4584383).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Fluss.HotChocolate/AddExtensionMiddleware.cs	52.94%	6 Missing and 2 partials ⚠️
src/Fluss/UnitOfWork/UnitOfWorkFactory.cs	91.30%	0 Missing and 2 partials ⚠️
src/Fluss/UnitOfWork/UnitOfWorkRecordingProxy.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
+ Coverage   80.64%   86.30%   +5.65%     
==========================================
  Files          83       83              
  Lines        3731     3732       +1     
  Branches      375      376       +1     
==========================================
+ Hits         3009     3221     +212     
+ Misses        648      418     -230     
- Partials       74       93      +19     

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