Implement ReadAsync in ZipAESStream, extra simple version

[Numpsy]

Possible extra trivial version of #576, just for testing and a potential quick fix (whereby it doesn't attempt to actually be async)

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[Numpsy]

@piksel Any thoughts on applying the simple approach as a quick fix for for #572, given that I don't think a 'full' async version would actually do async reads at this point unless PartialInputStream was made async as well (ref #589)?

(It'd be nicer to make everything more fully async, not sure if there's any thoughts of a timespan for a next maintenance release vs. doign that though)

[piksel]

Yeah, I agree. Having the "stub" async override in the code base until another way is implemented is definitively better than trying to solve this correctly before doing anything.

[codecov]

Codecov Report

Merging #579 (525555e) into master (7ed87d1) will increase coverage by 2.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #579      +/-   ##
==========================================
+ Coverage   70.96%   73.05%   +2.08%     
==========================================
  Files          68       68              
  Lines       13417     8724    -4693     
==========================================
- Hits         9522     6373    -3149     
+ Misses       3895     2351    -1544     

Impacted Files	Coverage Δ
...ICSharpCode.SharpZipLib/Encryption/ZipAESStream.cs	84.00% <100.00%> (+2.44%)	⬆️
...pZipLib/Core/Exceptions/StreamDecodingException.cs	60.00% <0.00%> (-6.67%)	⬇️
...pLib/Core/Exceptions/StreamUnsupportedException.cs	60.00% <0.00%> (-6.67%)	⬇️
.../Core/Exceptions/UnexpectedEndOfStreamException.cs	60.00% <0.00%> (-6.67%)	⬇️
src/ICSharpCode.SharpZipLib/Checksum/Adler32.cs	81.81% <0.00%> (-3.29%)	⬇️
...pCode.SharpZipLib/Zip/Compression/PendingBuffer.cs	74.13% <0.00%> (-3.14%)	⬇️
.../ICSharpCode.SharpZipLib/Core/FileSystemScanner.cs	48.73% <0.00%> (-2.91%)	⬇️
...e.SharpZipLib/Zip/Compression/InflaterDynHeader.cs	91.48% <0.00%> (-2.17%)	⬇️
...harpZipLib/Zip/Compression/Streams/OutputWindow.cs	67.74% <0.00%> (-2.07%)	⬇️
src/ICSharpCode.SharpZipLib/Core/PathFilter.cs	10.25% <0.00%> (-1.46%)	⬇️
... and 55 more

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