[WIP] Alternate approach to async reads in ZipAESStream

[Numpsy]

Alternate approach to #576 which tries taking the simpler approach of using the 'useAsync' idea that some MS libs use for sharing async/sync versions of a function instead of duplicating all the logic, in order to only have a single version of ReadAndTransform

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[codecov[bot]]

Codecov Report

Merging #624 (f8b19ed) into master (e1e1a91) will increase coverage by 0.05%. The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master     #624      +/-   ##
==========================================
+ Coverage   73.82%   73.87%   +0.05%     
==========================================
  Files          69       69              
  Lines        8291     8311      +20     
==========================================
+ Hits         6121     6140      +19     
- Misses       2170     2171       +1     

Impacted Files	Coverage Δ
...ICSharpCode.SharpZipLib/Encryption/ZipAESStream.cs	84.61% <92.85%> (+0.32%)	:arrow_up:
src/ICSharpCode.SharpZipLib/Core/StreamUtils.cs	51.78% <100.00%> (+5.78%)	:arrow_up:

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