src: use BaseObjectPtr to store SNI context

Rather than relying on a link to the JS object, store a pointer to
the C++ object directly.

PR-URL: https://github.com/nodejs/node/pull/30548


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>