Add ExecReload option to pgcat.service for configuration reloads

[dog-64]

This commit adds the ExecReload option to the pgcat.service file, allowing for reloading the PgCat service configuration without restarting the service. This update enables the use of sudo systemctl reload pgcat.service for applying configuration changes more efficiently.

Before the fix:

The service configuration did not support the reload command, necessitating a full restart to apply any configuration changes, which could cause downtime and interrupt active connections.

After the fix:

The service configuration now includes the ExecReload=/bin/kill -SIGHUP $MAINPID option, allowing for seamless configuration reloads:

[Unit]
Description=PgCat pooler
After=network.target
StartLimitIntervalSec=0

[Service]
User=pgcat
Type=simple
Restart=always
RestartSec=1
Environment=RUST_LOG=info
LimitNOFILE=65536
ExecStart=/usr/bin/pgcat /etc/pgcat.toml
ExecReload=/bin/kill -SIGHUP $MAINPID

[Install]
WantedBy=multi-user.target

[levkk]

There is also the option to just run pkill pgcat -HUP, so a restart isn't necessary, but yes, this is a good addition.