Add flushing support for gzip compressor

[sidhant007]

Uses Z_FULL_FLUSH at the point where flushing is done. Therefore it enables the user to read data between any two flush points.

Fixes 1656 for Gzip Filters. More context here

Relies on Z_BUF_ERROR to determine when to stop deflating.

[codecov]

Codecov Report

Merging #116 into develop will increase coverage by 0.04%.
The diff coverage is 72.22%.

@@             Coverage Diff             @@
##           develop     #116      +/-   ##
===========================================
+ Coverage    68.96%   69.00%   +0.04%     
===========================================
  Files           80       80              
  Lines         3444     3462      +18     
  Branches      1027     1033       +6     
===========================================
+ Hits          2375     2389      +14     
+ Misses         452      451       -1     
- Partials       617      622       +5     

Impacted Files	Coverage Δ
src/zlib.cpp	92.85% <ø> (ø)
include/boost/iostreams/filter/symmetric.hpp	71.87% <50.00%> (-2.55%)	⬇️
include/boost/iostreams/filter/gzip.hpp	60.85% <100.00%> (+0.46%)	⬆️
include/boost/iostreams/filter/zlib.hpp	75.00% <100.00%> (+2.11%)	⬆️
.../iostreams/detail/streambuf/indirect_streambuf.hpp	77.77% <0.00%> (ø)
.../iostreams/detail/adapter/non_blocking_adapter.hpp	76.47% <0.00%> (+5.88%)	⬆️

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[sidhant007]

Hey @jeking3, can you kindly review this PR

[R030t1]

Yeet this please
It is preeminently yeetable

Worth noting, I guess, that you can use new/delete to force the desired behavior.

Any chance to generalize this for the other compressors? I'll have a look at how hard it is to add zstd.