src: use std::unique_ptr for STACK_OF(X509)

Convert manual memory management of STACK_OF(X509) instances to
std::unique_ptr with a custom deleter.

Note the tasteful application of std::move() to indicate a function
that consumes its argument.

PR-URL: https://github.com/nodejs/node/pull/19087


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>