tls: type checking for `key`, `cert` and `ca` options

PR-URL: https://github.com/nodejs/node/pull/14807
Fixes: https://github.com/nodejs/node/issues/12802


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>