Decouple request context and trace header from package:appengine

[sigurdm]

Decouples request handling and trace propagation from package:appengine's ClientContext:

• X-Cloud-Trace-Context propagation (app/lib/shared/utils.dart, app/lib/shared/handler_helpers.dart):
  • Extract the 32-character hex trace ID from X-Cloud-Trace-Context at the start of _logRequestWrapper and bind it to the request zone via withTraceId.
  • Read currentTraceId in cloudTraceHeaders() and _logRequestWrapper error responses instead of package:appengine's context.traceId.
• HTTPS wrapper & FakeClientContext (app/lib/shared/handler_helpers.dart, app/lib/service/services.dart):
  • Use !envConfig.isRunningLocally instead of context.isProductionEnvironment in _httpsWrapper and remove FakeClientContext.