unlock with PIN (#8977)

* add custom password to unlock settings

* If not set, use admin password; if set, use custom settings password.
* At least 4 characters.
* Set with gui or command line.

Signed-off-by: 21pages <sunboeasy@gmail.com>

* Update cn.rs

---------

Signed-off-by: 21pages <sunboeasy@gmail.com>
Co-authored-by: RustDesk <71636191+rustdesk@users.noreply.github.com>

flutter/lib/web/bridge.dart

@@ -1622,5 +1622,13 @@ class RustdeskImpl {
     throw UnimplementedError();
   }
 
+  String mainGetUnlockPin({dynamic hint}) {
+    throw UnimplementedError();
+  }
+
+  String mainSetUnlockPin({required String pin, dynamic hint}) {
+    throw UnimplementedError();
+  }
+
   void dispose() {}
 }