feat: add EventSubscriber trait and inmemory impl

[ar3s3ru]

Closes #92

[codecov]

Codecov Report

Merging #94 into master will increase coverage by 4.78%.
The diff coverage is 97.19%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
+ Coverage   47.16%   51.95%   +4.78%     
==========================================
  Files          13       13              
  Lines         600      666      +66     
==========================================
+ Hits          283      346      +63     
- Misses        317      320       +3     

Flag	Coverage Δ
#unit	51.95% <97.19%> (+4.78%)	⬆️

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Impacted Files	Coverage Δ
eventually-postgres/src/lib.rs	0.00% <ø> (ø)
eventually-util/src/inmemory.rs	92.13% <97.19%> (+1.34%)	⬆️

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[sonarqubecloud]

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