Cleanup CMake makefiles for nlohmann_json.

[marcalff]

Fixes #1910

Changes

Moved nlohmann_json dependency logic to cmake/nlohmann-json.cmake

No behavior change.

Added documentation for various ways to add the nlohmann_json dependency.

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed (N/A)
• Tested local build using a package
• Tested local build using git submodules
• Tested local build using github download

[codecov]

Codecov Report

Merging #1912 (c1f91e0) into main (8a9d085) will decrease coverage by 0.06%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1912      +/-   ##
==========================================
- Coverage   85.78%   85.72%   -0.05%     
==========================================
  Files         171      171              
  Lines        5252     5252              
==========================================
- Hits         4505     4502       -3     
- Misses        747      750       +3     

Impacted Files	Coverage Δ
ext/src/http/client/curl/http_client_curl.cc	80.31% <0.00%> (-1.13%)	⬇️

[lalitb]

LGTM. Thanks you, this make root CMakeLists.txt much cleaner.