MEDIUM: ssl/ech: config and load keys

MEDIUM: ssl/ech: config and load keys

This patch introduces the USE_ECH option in the Makefile to enable
support for Encrypted Client Hello (ECH) with OpenSSL.

A new function, load_echkeys, is added to load ECH keys from a specified
directory. The SSL context initialization process in ssl_sock.c is
updated to load these keys if configured.

A new configuration directive, `ech`, is introduced to allow users to
specify the ECH key  directory in the listener configuration.