crypto: add getIntOption function to reduce dupl

This commit adds a getIntOption function to reduce the code duplicated
for getting the padding, and saltLength options.

PR-URL: https://github.com/nodejs/node/pull/20247


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>