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

❌ Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.00%. Comparing base (601244b) to head (c7230cc).
⚠️ Report is 79 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/iostreams/filter/symmetric.hpp	50.00%	1 Missing and 4 partials ⚠️

Additional details and impacted files

@@             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     

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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.

[subconscious0]

Hi can @jeking3 or someone kindly review this PR?