add check password hook

its main use case is to allow to easily support things like password+OTP for
protocols without keyboard interactive support such as FTP and WebDAV

sftpgo.json

@@ -84,7 +84,9 @@
     "credentials_path": "credentials",
     "pre_login_hook": "",
     "post_login_hook": "",
-    "post_login_scope": 0
+    "post_login_scope": 0,
+    "check_password_hook": "",
+    "check_password_scope": 0
   },
   "httpd": {
     "bind_port": 8080,
@@ -101,4 +103,4 @@
     "ca_certificates": [],
     "skip_tls_verify": false
   }
-}
+}