add a new configuration section for HTTP clients

HTTP clients are used for executing hooks such as the ones used for custom
actions, external authentication and pre-login user modifications.

This allows, for example, to use self-signed certificate without defeating the
purpose of using TLS

sftpgo.json

@@ -65,5 +65,9 @@
     "auth_user_file": "",
     "certificate_file": "",
     "certificate_key_file": ""
+  },
+  "http": {
+    "timeout": 20,
+    "ca_certificates": []
   }
 }