fix: make zlib a private dependency

[Tachi107]

Before this patch, the zlib.h header was included by pistache/http.h, meaning that users had to install the development headers on zlib to build projects which depend on Pistache (in other terms, it was a transitive, public, API dependency). Since zlib.h was included by http.h just to get the value of zlib's default compression level, I simply hardcoded it to 6; it's unlikely to ever change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.09%. Comparing base (31bc54d) to head (b381c22).
Report is 217 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1176   +/-   ##
=======================================
  Coverage   78.08%   78.09%           
=======================================
  Files          53       53           
  Lines        7077     7075    -2     
=======================================
- Hits         5526     5525    -1     
+ Misses       1551     1550    -1     

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[kiplingw]

But is this a good idea? Is there any particular reason why the include needs to be removed? Was it having a noticeable impact on preprocessor burden at compile time? You are right that the constant is unlikely to change, but it still could. zlib has gone through many changes and likely still will.

[kiplingw]

Hey @Tachi107, do you want me to close this or do you need more time?

[Tachi107]

Adding a normalized 1..0 value independent of the underlying compression algorithm used seems like a better solution (which still needs to be implemented). Closing.