Add SSL/TLS traffic decryption for PostgreSQL backend connections (libpq)

[rahim-kanji]

Description:

ProxySQL currently supports SSL traffic decryption using an NSS keylog file for:

• frontend connections (client → ProxySQL), and
• MySQL backend connections.

However, SSL-encrypted traffic between ProxySQL and PostgreSQL backends is not currently decrypted.

This issue proposes extending the existing NSS keylog–based decryption support to PostgreSQL backend connections by integrating it on the libpq side.

Expected Behavior:

• When an NSS keylog file is configured, ProxySQL should be able to decrypt SSL traffic between ProxySQL and PostgreSQL backends.
• The PostgreSQL backend implementation should follow the same approach used for MySQL backend SSL/TLS decryption.