[SDK] enable deriving from ResourceDetector to create a Resource

[dbarker]

Allow derived classes from ResourceDetector to create a resource.

Fixes #3246

Changes

• Makes ResourceDetector a friend of Resource (replacing OTELResourceDetector)
• Adds a protected Create method to ResourceDetector that calls the constructor of Resource
• Adds test case for a derived resource detector

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	414b4e3
🔍 Latest deploy log	https://app.netlify.com/sites/opentelemetry-cpp-api-docs/deploys/678937ccca94da00080a73ce

[marcalff]

LGTM, thanks for the fix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.71%. Comparing base (465cb4a) to head (414b4e3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3247      +/-   ##
==========================================
+ Coverage   87.71%   87.71%   +0.01%     
==========================================
  Files         198      198              
  Lines        6271     6273       +2     
==========================================
+ Hits         5500     5502       +2     
  Misses        771      771              

Files with missing lines	Coverage Δ
sdk/include/opentelemetry/sdk/resource/resource.h	100.00% <ø> (ø)
...ude/opentelemetry/sdk/resource/resource_detector.h	100.00% <ø> (ø)
sdk/src/resource/resource_detector.cc	100.00% <100.00%> (ø)