google_cloud_shelf: deprecate cloudTraceContextHeader in favor of traceparentHeader

In `pkgs/google_cloud_shelf/lib/src/constants.dart`, `cloudTraceContextHeader` is explicitly defined as `'traceparent'` (W3C trace context header). However, the constant name `cloudTraceContextHeader` implies legacy `X-Cloud-Trace-Context` headers.

Since `cloudTraceContextHeader` is part of the public API exported by `google_cloud_shelf.dart`, we should:
1. Introduce a new constant `traceparentHeader` (or `w3cTraceparentHeader`).
2. Deprecate `cloudTraceContextHeader` in favor of the new constant.

Start in pkgs/google_cloud_shelf/lib/src/constants.dart and inspect the public export in google_cloud_shelf.dart. Follow the package's existing deprecation conventions, add the replacement traceparent constant, and deprecate cloudTraceContextHeader while preserving the public API. Done means the new name represents the W3C traceparent header and the old name is clearly marked for replacement.

Written by the indexing model from the issue text.