Move env-var read logic to common dir, and optional reading of env-var

[lalitb]

Changes

1. Move env var reading logic to sdk::common namespace, this allows it to be reused at multiple places.
2. Optional reading of env variables based on macro -DNO_GETENV. The env var is not read if the macro is set. This allows using sdk with platforms like UWP which doesn't support env variables. ( as discussed in Allow to programmatically populate settings for OTELResourceDetector #656 ).
3. Make required otlp-grpc exporter options to be read through env variable, which further allows loading this exporter at run-time using the dynamic-load plugin. This would be further added for other exporters also.

Please provide a brief description of the changes here.

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 #924 (b1fe988) into main (a9b2f9f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #924      +/-   ##
==========================================
+ Coverage   95.36%   95.36%   +0.01%     
==========================================
  Files         159      160       +1     
  Lines        6775     6779       +4     
==========================================
+ Hits         6460     6464       +4     
  Misses        315      315              

Impacted Files	Coverage Δ
sdk/test/resource/resource_test.cc	93.46% <ø> (ø)
...k/include/opentelemetry/sdk/common/env_variables.h	100.00% <100.00%> (ø)
sdk/src/resource/resource_detector.cc	100.00% <100.00%> (ø)

[ThomsonTan]

Convert it to lower case before comparison, to make the comparison case insensitive?

[lalitb]

Good point will change that.

[lalitb]

Ok. I realize there is no method available in stdlib to perform this conversion. we have to iterate over the string to convert it. The existing comparison looks simple as compared to that :)

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Lalit Kumar Bhasin (511d941, 125d9b5, 3b2d300, 277e2d4, fbb046a, b130e87, 364c433, b51bdf6, b1fe988)

[lalitb]

/easycla