[IDEA] Make a bridge to the PHP-FPM

[rustatian]

After discussion with @wolfy-j .
Support a PHP-FPM (https://www.php.net/manual/en/install.fpm.php) process manager. Since we have the ability to redirect any input from any plugin (which uses workers and Pool implementation) to any external system, we can design and implement a new Pool that will redirect input to the PHP-FPM workers instead of RR workers.
TODO:

1. Implement Pool interface, create a new dir inside the pkg folder named fpm.
2. Add a new configuration section, to have the ability to use php-fpm pool impl.
3. We can use server plugin and implement a new, let's say, relay.

Useful links:
https://www.nginx.com/resources/wiki/start/topics/examples/phpfcgi/
https://github.com/php/php-src/tree/master/sapi/fpm/fpm

[tarampampam]

@rustatian Often nginx is not only reverse proxy before the app and static assets - it is used for common HTTP header management (cache-control), proxying requests to other upstream, redirects, etc. So, if RoadRunner will be used for an Nginx replacement - we should implement a lot of Nginx features in RoadRunner. So, we really want it?

[rustatian]

@tarampampam Yeah, I am pretty sure, that we shouldn't re-implement nginx here 😄 Ok, so, if the community requested a bridge like that, we will continue the discussion.