src: refactor default trace writer out of agent

The agent code is supposed to manage multiple writers/clients.
Adding the default writer into the mix breaks that encapsulation.
Instead, manage default options through a separate "virtual"
default client handle, and keep the file writer management
all to the actual users.

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>