Add a flag/config option to enable opcache on the CLI: phpbench --php-opcache

• Overrides the PHP config to opcache.enable_cli
• Sets opcache.file_cache directory and opcache.file_cache_only (is "only" needed?)

TODO:

• Current opcache "status" is misleading as it will be ✔️ when opcache.enable is true, but on CLI there is no opcache unless enable_cli is set.
• allow opcache to be disabled as well as enabled