refactor: use libcurl in HTTP testing examples

[coryan]

The next test will receive a URL, and we do not have reasonably easy
ways to parse a URL. None that can be put in an example anyway: we
could use a regexp, but they are brutally complicated.

[codecov]

Codecov Report

Merging #191 (7d07d7f) into main (8cb15c4) will decrease coverage by 0.12%.
The diff coverage is 82.05%.

@@            Coverage Diff             @@
##             main     #191      +/-   ##
==========================================
- Coverage   97.54%   97.42%   -0.13%     
==========================================
  Files          65       65              
  Lines        1551     1556       +5     
==========================================
+ Hits         1513     1516       +3     
- Misses         38       40       +2     

Impacted Files	Coverage Δ
...xamples/site/testing_http/http_integration_test.cc	87.03% <82.05%> (-2.76%)	⬇️

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