src: use unique pointer for tracing_agent

Use std::unique_ptr instead of raw pointers for the
tracing_agent_ in node.cc. This makes ownership clearer and we
don't risk a memory leak.

PR-URL: https://github.com/nodejs/node/pull/17012


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>