feat(propagation): Support environment variable propagation

[9999years]

Fixes: #3285

Changes

Adds opentelemetry::propagation::environment::{EnvironmentExtractor, EnvironmentInjector} to propagate context across process boundaries, gated on an unstable experimental_propagation_env_carrier feature flag, matching the Python/Java/Go SDKs.

This can be used like:

let context: Context = TraceContextPropagator::new().extract(&EnvironmentExtractor::new());

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 93.07692% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.2%. Comparing base (95c1d4b) to head (2629cdf).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry/src/propagation/environment.rs	93.0%	9 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #3587    +/-   ##
======================================
  Coverage   83.2%   83.2%            
======================================
  Files        130     131     +1     
  Lines      28232   28362   +130     
======================================
+ Hits       23491   23612   +121     
- Misses      4741    4750     +9     

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[github-actions]

Thank you for your contribution! This PR has been automatically marked as stale because it has not had activity in the last 14 days. This may be due to a delay in review on our side or awaiting a response from you; either is fine, and we appreciate your patience.

It will be closed in 14 days if no further activity occurs. Pushing a new commit or leaving a comment will remove the stale label and keep the PR open.