src: use unique_ptr for requests in crypto

Instead of raw pointerns, use std::unique_ptr for PBKDF2Request and
RandomBytesRequest. This makes ownership more clear.

PR-URL: https://github.com/nodejs/node/pull/17000


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>