feat(api): add experimental env var propagators

[pellared]

Fixes open-telemetry#3285
Design discussion issue (if applicable) N/A

Changes

This adds experimental support for the environment-variable carrier specification so Rust applications can propagate context across parent/child process boundaries without relying on network transports.

• Add EnvVarExtractor and EnvVarInjector under opentelemetry::propagation, gated behind otel_unstable
• Normalize environment variable names according to the latest spec for Get, Set, and Keys, while keeping the carrier format-agnostic and value-opaque
• Document the new API in the crate docs, README, and changelog, and add examples/env-var-propagation to show parent/child process propagation with TraceContextPropagator
• Follow the latest operational guidance by treating extraction as startup-time input and using a child-process environment copy instead of mutating the parent process environment

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)

[pellared]

Closing in favor of the upstream draft PR at open-telemetry#3574.