Fix nlohmann_json package dependency

[lalitb]

Fixes # 981

• The nlohmann_json package should be required only when the component using it are part of the build, else be ignored.
• Fail with an appropriate error if this package is missing, based on whether the build is initiated from opentelemetry-cpp git repo or release tarball.
• Make zpage and test for validating w3c trace-context optional in build, as they both use nlohmann_json.

For significant contributions please make sure you have completed the following items:

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

[codecov]

Codecov Report

Merging #1017 (915cbbc) into main (bfab71a) will decrease coverage by 0.58%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1017      +/-   ##
==========================================
- Coverage   95.41%   94.83%   -0.57%     
==========================================
  Files         161      151      -10     
  Lines        6833     5972     -861     
==========================================
- Hits         6519     5663     -856     
+ Misses        314      309       -5     

Impacted Files	Coverage Δ
sdk/src/trace/span.cc	81.71% <0.00%> (-9.75%)	⬇️
...include/opentelemetry/sdk/trace/multi_recordable.h	88.00% <0.00%> (-6.00%)	⬇️
...include/opentelemetry/sdk/common/circular_buffer.h	97.92% <0.00%> (-2.08%)	⬇️
ext/test/zpages/tracez_data_aggregator_test.cc
...de/opentelemetry/ext/zpages/threadsafe_span_data.h
ext/test/zpages/tracez_processor_test.cc
ext/include/opentelemetry/ext/zpages/tracez_data.h
ext/src/zpages/tracez_processor.cc
...nclude/opentelemetry/ext/zpages/tracez_processor.h
ext/src/zpages/tracez_data_aggregator.cc
... and 3 more

[ThomsonTan]

If the repo is cloned without submodule, will this work?

[lalitb]

I have now modified the second if condition to make it more explicit.

Error message if cloned without submodules,

  nlohmann_json package was not found.  Please either provide it manually or
  clone with submodules.  To initialize, fetch and checkout any nested
  submodules, you can use the following command:

  git submodule update --init --recursive

Error message if using release tar-ball:

CMake Error at CMakeLists.txt:290 (find_package):
  By not providing "Findnlohmann_json.cmake" in CMAKE_MODULE_PATH this
  project has asked CMake to find a package configuration file provided by
  "nlohmann_json", but CMake did not find one.

  Could not find a package configuration file provided by "nlohmann_json"
  with any of the following names:

    nlohmann_jsonConfig.cmake
    nlohmann_json-config.cmake

  Add the installation prefix of "nlohmann_json" to CMAKE_PREFIX_PATH or set
  "nlohmann_json_DIR" to a directory containing one of the above files.  If
  "nlohmann_json" provides a separate development package or SDK, be sure it
  has been installed.