process: add direct access to rss without iterating pages

Accessing the rss value through memoryUsage() can be expensive
because this method will also generate  memory usage statistics
by iterating on each page.
This commit intend to offer a more direct access to rss value.

Refs: #33384

PR-URL: https://github.com/nodejs/node/pull/34291


Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: James M Snell <jasnell@gmail.com>