feat(gzip): add GzipOutputStream async support

[piksel]

Follow up to #574 that implements DisposeAsync and "true" async write support to GzipOutputStream.

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[codecov[bot]]

Codecov Report

Merging #672 (e61251a) into master (519ed73) will increase coverage by 0.10%. The diff coverage is 90.32%.

@@            Coverage Diff             @@
##           master     #672      +/-   ##
==========================================
+ Coverage   74.60%   74.70%   +0.10%     
==========================================
  Files          72       72              
  Lines        8457     8465       +8     
==========================================
+ Hits         6309     6324      +15     
+ Misses       2148     2141       -7     

Impacted Files	Coverage Δ
...ib/Zip/Compression/Streams/DeflaterOutputStream.cs	82.47% <ø> (+7.70%)	:arrow_up:
...c/ICSharpCode.SharpZipLib/GZip/GzipOutputStream.cs	86.48% <90.32%> (-1.02%)	:arrow_down:

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