Adding support for SSLKEYLOGFILE in the frontend

Hello folks,

Attached is a patch to add support for logging secrets used in TLS
connection after psql is initialized. This adds a new env var
SSLKEYLOGFILE on the client side that points to a text file where keys
will be logged. If a user runs psql multiple times with the same
SSLKEYLOGFILE, new entries will be appended to that file. There is no
change in behavior if that env var is not set or set to an empty
string. This is useful for cases when a client wants to analyze TCP
packets using a tool like wireshark while using TLS. This will enable
wireshark to decrypt the packets and decode as postgres wire protocol
messages. I did not add this to the backend because I thought using
wireshark is more common on the frontend.

The keylogfile format is documented here
https://www.ietf.org/archive/id/draft-thomson-tls-keylogfile-00.html

Example usage:

root(at)guest:~/postgres# SSLKEYLOGFILE=./key.txt
/usr/local/pgsql/bin/psql
"postgresql://user:pass(at)host:5432/postgres?sslmode=require"
psql (18devel, server 17.2)
SSL connection (protocol: TLSv1.3, cipher: TLS_AES_256_GCM_SHA384,
compression: off, ALPN: postgresql)
Type "help" for help.

postgres=>
\q
root(at)guest:~/postgres# cat key.txt
SERVER_HANDSHAKE_TRAFFIC_SECRET ****
EXPORTER_SECRET ****
SERVER_TRAFFIC_SECRET_0 ****
CLIENT_HANDSHAKE_TRAFFIC_SECRET ***
CLIENT_TRAFFIC_SECRET_0 ****

A few things I am not sure about:
1. Where should I add automated tests and docs for this? I did not see
any unit tests for the surrounding functions.
2. Should I use perror to report error here? I did not want to use
libpq_append_conn_error because this is not a connection related
error.

Please let me know if I can clarify anything.

--
Thanks and regards
Abhishek Chanda