Implement the FastCGI protocol

## What is the problem your feature solves, or the need it fulfills?

Implement the FastCGI protocol.

## Describe the solution you'd like

Implement FastCGI protocol for PHP so that pingora can use PHP-FPM as backend.

## Describe alternatives you've considered

Currently, Nginx and its derivatives support the FastCGI protocol, among which Nginx + FastCGI + PHP-FPM is the mainstream environment.

## Additional context

https://en.wikipedia.org/wiki/FastCGI
https://www.php.net