Stackdriver - adding support for Google X-Cloud-Trace-Context propagation format

[ivan-brko]

Fixes

Issue https://github.com/open-telemetry/opentelemetry-rust-contrib/issues/3

Changes

Added GoogleTraceContextPropagator to opentelemetry-stackdriver crate, behind a feature flag propagator. Implementation is based on the official GCP golang library implementation and Rust opentelemetry implementation of W3C Propagator.

Merge requirement checklist

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3827119) 40.4% compared to head (f537a66) 40.4%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #25 +/- ## ===================================== Coverage 40.4% 40.4% ===================================== Files 12 12 Lines 1279 1279 ===================================== Hits 517 517 Misses 762 762 ```

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