src: improve KVStore API

This adds `const char*` based APIs to KVStore to avoid multiple string
conversions (char -> Utf8 -> Local -> char etc.) when possible.

PR-URL: https://github.com/nodejs/node/pull/31773


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>