add XOAUTH2

start the countdown, let's see how long it takes for your favorite
Go-based proprietary SFTP server to notice this change, copy the SFTPGo
code and thus violate its license, and announce the same feature :)

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

sftpgo.json

@@ -407,7 +407,14 @@
     "encryption": 0,
     "domain": "",
     "templates_path": "templates",
-    "debug": 0
+    "debug": 0,
+    "oauth2": {
+      "provider": 0,
+      "tenant": "",
+      "client_id": "",
+      "client_secret": "",
+      "refresh_token": ""
+    }
   },
   "plugins": []
 }