Update Env var getter / setter to reflect spec changes

[jack-berg]

See open-telemetry/opentelemetry-specification#4961

Changes:

• EnvironemtnGetter#keys() normalizes results and logs a one time warning explaining that it may not work with certain propagators
• EnvironmentGetter / EnvironmentSetter treat values as opaque, skipping any detection / normalization related to characters or size. See Decouple the responsibilities of the environment variable propagation carrier opentelemetry-specification#4961 (comment)
• Update key normalization to reflect spec guidance:
  • Convert ASCII letters to uppercase
  • Any non-ASCII letter, digit, or underscore is replaced with underscore
  • Add a _ prefix if the key would start with a digit

cc @Bhagirath00

[pellared]

Per open-telemetry/opentelemetry-specification#4944

[codecov]

Codecov Report

❌ Patch coverage is 81.48148% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.30%. Comparing base (7f50d5b) to head (896ca16).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
...y/api/incubator/propagation/EnvironmentSetter.java	76.92%	1 Missing and 2 partials ⚠️
...y/api/incubator/propagation/EnvironmentGetter.java	85.71%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8233      +/-   ##
============================================
- Coverage     90.32%   90.30%   -0.02%     
- Complexity     7653     7655       +2     
============================================
  Files           843      842       -1     
  Lines         23080    23083       +3     
  Branches       2312     2317       +5     
============================================
  Hits          20846    20846              
  Misses         1516     1516              
- Partials        718      721       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

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