[CMAKE] find or fetch nlohmann-json

[dbarker]

Fixes #3190

• The otlp targets no longer have a public dependency on nlohmann-json targets.

Fixes #1448

• nlohmann-json will now be fetched and install if 1) a nlohmann-json package was not found and 2) OPENTELEMETRY_INSTALL=ON

Changes

• Update cmake/nlohmann-json.cmake to support finding an installed package or using FetchContent on the git submodule or github repo.
• Removes the nlohmann_json_clone target (from the ExternalProject approach) and any dependencies on it.

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

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
🔨 Latest commit	16130bb
🔍 Latest deploy log	https://app.netlify.com/projects/opentelemetry-cpp-api-docs/deploys/686c0e89ee4e1e000878f21b

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.96%. Comparing base (711625f) to head (16130bb).
⚠️ Report is 280 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3523   +/-   ##
=======================================
  Coverage   89.96%   89.96%           
=======================================
  Files         219      219           
  Lines        7051     7051           
=======================================
  Hits         6343     6343           
  Misses        708      708           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[owent]

LGTM, thanks.

[marcalff]

LGTM