[php-src] Issue #13533: Add PHP_FCGI_MAX_IDLE_TIMEOUT envairoment variabile option for php-cgi

Issue: https://github.com/php/php-src/issues/13533 Author: ducktype ### Description I'm writing an "external" multiplatform php fastcgi process manager in golang for the caddy webserver, something like php-fpm but using the php-cgi executable in fastcgi mode; while implementing it using socket passing to be faster simpler and zero-copy between processes i've found that i wanted to add an option in my process manager analogous to php-fpm pm.process_idle_timeout to free memory when the server load is low and to avoid eventual Memory leaks I've thinked about how to implement this feature for a while but without finding a solution keeping zero-copy socket passing that does bot require modifing php-cgi itself. The proposal is to add an envairoment variabile option that instruct php-cgi processes to self terminate after a certain amount of time without receiving requests something like the already existing PHP_FCGI_MAX_REQUESTS that already aligns with php-fpm pm.max_requests. Eventually having millisecond granularity unlike second granularity only like in pm.process_idle_timeout will be a plus. That Is... i'm not missing something obviuos and that the proposal make sense.