support for FastCGI over TLS

Description

Currently, php-fpm supports UNIX and TCP socket operation. However, to enhance security (and privacy of data in transit) it might be useful to also have an option for a TLS socket, ideally mTLS, additional to TCP for remote connections.

In such cases, it is already possible to use transparent transport encryption (VPN, or a TLS-tunnel), but php-fpm wouldn't have an idea about it. With integrated TLS support, information from the TLS session (e.g. X.509 attributes) could be used to populate "allowed_clients" or php-fpm's ENV to control further behavior.

Since (optional) OpenSSL support is already present in PHP, it should be relatively trivial to also support a TLS listener - with a bit more work for mTLS and actually making use of TLS session information.