add a global whitelist

if defined only the listed IPs/networks can access the configured
services, all other client connections will be dropped before they
even try to authenticate

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

sftpgo.json

@@ -17,6 +17,7 @@
     "data_retention_hook": "",
     "max_total_connections": 0,
     "max_per_host_connections": 20,
+    "whitelist_file": "",
     "defender": {
       "enabled": false,
       "driver": "memory",