google_cloud_shelf: deprecate cloudTraceContextHeader in favor of traceparentHeader

[kevmoo]

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.

[kevmoo]

Found during #298