src: reduce unnecessary includes

A forward declaration suffices and means that the tracing
agent header and its dependencies don’t need to be included
in nearly every Node.js file.

PR-URL: https://github.com/nodejs/node/pull/21867


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>