Cleanup CMake makefiles for CURL usage

[marcalff]

Fixes #1913

Changes

General CMake cleanup related to CURL:

• Removed multiple find_package(CURL), used find_package(CURL) from only one place in the top level makefile,
• Removed if(CURL_FOUND) tests, which are semantically incorrect. A given library should be compiled when there is a reason to build it, not because CURL happens to be available. This also make the build more robust in a super build, when unrelated packages can use CURL and set CURL_FOUND for unrelated reasons,
• Fixed the W3C trace test build, which did not honor the build option,
• Fixed the WITH_OTLP_HTTP option to be predictable,
• Added a build option for HTTL client/server examples, there was previously no way to control the build of these artifacts.

See the CHANGELOG Important changes section for more details.

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1916 (8e17b6c) into main (46b16ec) will increase coverage by 0.75%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1916      +/-   ##
==========================================
+ Coverage   86.16%   86.90%   +0.75%     
==========================================
  Files         171      165       -6     
  Lines        5257     4594     -663     
==========================================
- Hits         4529     3992     -537     
+ Misses        728      602     -126     

Impacted Files	Coverage Δ
sdk/src/trace/batch_span_processor.cc	90.70% <0.00%> (-0.77%)	⬇️
api/include/opentelemetry/nostd/string_view.h	97.96% <0.00%> (-0.08%)	⬇️
...ntelemetry/ext/http/client/curl/http_client_curl.h
...lemetry/ext/http/client/curl/http_operation_curl.h
...t/src/http/client/curl/http_client_factory_curl.cc
ext/src/http/client/curl/http_operation_curl.cc
...nclude/opentelemetry/ext/http/client/http_client.h
ext/src/http/client/curl/http_client_curl.cc

[owent]

LGTM

[owent]

Do we need also add -DWITH_EXAMPLES_HTTP=ON in at least one CI job? Or we can not ensure the http example can be compiled success.

[marcalff]

Do we need also add -DWITH_EXAMPLES_HTTP=ON in at least one CI job? Or we can not ensure the http example can be compiled success.

Good point, looking into this.

[marcalff]

Do we need also add -DWITH_EXAMPLES_HTTP=ON in at least one CI job? Or we can not ensure the http example can be compiled success.

Good point, looking into this.

Resolved, tests build in maintainer mode.

[lalitb]

LGTM with nit comments :)

[esigo]

LGTM
Thanks for the fix :)

[marcalff]

@owent @lalitb @esigo

Thanks all for the review.
All comments addressed, now ready to merge.